If you're considering making a big cash contribution to a charity and deducting it this year, Congress may have given you an unusual opportunity.

At a time when lawmakers and the Internal Revenue Service have tightened up on such things as donations of cars and real estate facade easements, they have also created several special breaks for charitable gifts after Hurricane Katrina. Most of them have to be related to Katrina, but one key one does not, and it could be a major benefit for taxpayers willing and able to give away big bucks.

To put this into context, you have to understand the various limitations on charitable giving that normally determine whether you can take a writeoff and, if you can, just how big it normally can be.

First, you have to itemize your deductions to get a break. This provision, which is frequently criticized, means that many lower-income donors get no tax benefit for their gifts.

Second, for itemizers, there are broad rules limiting the amount by which you can reduce your taxable income by making gifts.

In a normal year -- and this one isn't -- your charitable deductions cannot exceed 50 percent of your adjusted gross income. In fact, depending on what you give and to whom you give it, the limit can be lower. The 50 percent limit applies for gifts to churches, schools, hospitals and public charities. Because of the limit, these are known colloquially as "50 percent organizations," and the IRS lists them in Publication 78, which is available on its Web site, www.irs.gov.

But there is a further limit: If you donate appreciated property, such as stocks that have gone up in value since you bought them, your deduction is limited to 30 percent of your AGI.

That isn't all, of course. If your gift is to a charity other than a 50 percent organization -- and these include veterans groups, fraternal societies, certain private foundations and others -- the limit on your deduction is 30 percent of your AGI. And the deduction of donations of appreciated property to a 30 percent organization is limited to 20 percent of your AGI.

A taxpayer who hits one of these limits can carry unused deductions forward for up to five years, deducting them later, but is subject to the same percentage limits.

However, in the wake of Katrina, Congress temporarily changed the rules. This year, you may deduct donations made to most 50 percent organizations up to 100 percent of your AGI.

The donation has to be cash, and it has to have been made between Aug. 28 and Dec. 31 of this year. Also, noted Martin Nissenbaum, national director of personal income-tax planning at accountants Ernst & Young, the recipient cannot be a supporting organization, a special kind of charity whose purpose is to aid another, public charity. Nor can it be a donor-advised fund, a charitable mutual fund to which a person may contribute and then later recommend causes to which his or her money should go.

"Congress wised up to that. They weren't going to let people slam money into those" types of organizations to hand out later, Nissenbaum said.

On the other hand, he said, donations do not have to be for Hurricane Katrina relief, unless the donor is a business.

The normal rules were enacted in part to keep wealthy people from possibly wiping out their tax liabilities with charitable gifts, but lawmakers were apparently more interested this year in eliciting money for storm victims than in holding down deductions. As a result, they offered a big break for those who can afford it in a bill enacted in September.

It may be tough to imagine someone able to give away cash equal to his entire AGI, but Nissenbaum said, "while it's unusual, there are quite a few."

For example, he said, a wealthy person who gets a large share of his income from tax-exempt bonds might be able to give away enough cash to wipe out the taxes he would otherwise have to pay on the taxable portion of his income.

"There are people who would be quite happy to give away 100 percent of their [taxable] income," Nissenbaum said.

The new law also exempts charitable donations made under the new law from the broad restriction, known as the Pease provision after its legislative author, that kicks in on itemized deductions when a taxpayer's AGI hits $145,950.

The same bill that so generously eases the deduction restrictions also contains several other benefits, though these are specifically tied to charitable activities directly related to Katrina.

Taxpayers who take hurricane evacuees into their homes for at least 60 days are allowed an additional $500 personal exemption for each person they take in, up to a maximum of four people, or $2,000. The exemptions are allowable this year in the 60 days ended Dec. 31. If the period ends later, the exemption is available in 2006.

Lawmakers also boosted the mileage deduction for use of a car in charitable Katrina relief work from the current 14 cents a mile to 70 percent of the business-deduction rate. That works out to 34 cents at present. And they allowed a volunteer who uses his car in Katrina relief work to exclude from taxable income reimbursements from a charity for up to the business-mileage rate, currently 48.5 cents.

Though private forecasters worked these out weeks ago, the IRS recently made official the increases that inflation will cause in many areas of the tax system for 2006. Among them:

* The value of each personal and dependency exemption will be $3,300, up $100 from this year.

* The new standard deduction will be $10,300 for married couples filing a joint return, $5,150 for singles and $7,550 for heads of household.

* Tax-bracket thresholds will increase for each filing status.

* The annual gift-tax exemption will be $12,000, up from $11,000 in 2005.