Last week this column dealt with changes in tax rates for the years 1981 through 1984. Starting with 1985, the new tax law requires that individual rates be indexed to the consumer price index.

Actually the tax rates as expressed in percentages will not be changed. Instead, the upper and lower limits of each tax bracket will be adjusted upward each year to reflect the annual change in the CPI for the 12-month period ending Sept. 30 of the preceding year.

Interestingly, the tax act assumes continuing inflation, although not necessarily at a high rate. The law calls for increasing the bracket limits, but makes no provision for a decrease if a future year should, for whatever reason, bring a reduction in the CPI.

In addition to the tax-bracket limits, the personal exemption -- now $1,000 -- and the zero bracket amounts will be similarly indexed. They will be increased by the same cost-of-living adjustment determined for the bracket changes.

Indexing's purpose is to eliminate what has been referred to as bracket creep. Without indexing, a taxpayer who gets a wage increase each year equal to or greater than the CPI increase may end up losing in terms of purchasing power because the wage increase may mean a move into a higher tax bracket.

Under the new procedure, the bracket limits, personal exemptions and zero bracket amount will go up in step with annual increases in the cost of living as reflected in the CPI.

So if your income goes up at the same rate as the CPI, at least you will be spared the trauma of moving into a higher tax bracket without realizing any increase in the real value of your income. Marriage penalty

Married couples with both spouses employed generally pay more income tax than if they were single with the same respective incomes. Under the new law, such couples will get a tax break in addition to the general cuts.

In 1982, a two-earner couple filing a joint return will be able to deduct 5 percent of the qualifying earned income of the spouse with the lower earnings, up to a ceiling of $1,500 (income of $30,000). Starting in 1983 the deduction will go to 10 percent and the ceiling to $3,000.

Before figuring the credit, earned income will have to be reduced by some exclusions, primarily employe business expenses and Keogh or IRA contributions.

The new rule won't take all the sting out of the so-called marriage penalty, but it will bring the joint tax down closer to the combined tax liability of two single individuals.

However, couples with only one wage earner may be a little unhappy. Next year they will find themselves paying more tax than a couple with the same total family income but earned by both spouses.

This new differential may be particularly resented by people in a number of occupations -- the ministry, foreign service, the military, to mention just a few -- where the contributions of both spouses are generally considered equally important to good job performance. Maximum tax

Unearned income -- that is, income from interest and dividends, and similar payments not directly associated with personal labor -- is currently subject to a maximum tax rate of 70 percent.

On the other hand, there is a cap of 50 percent on earned income such as wages, salary, earnings from self-employment and retirement pay.

Under the new law, effective in 1982 the distinction between earned and unearned income is eliminated, and the top tax on income from any source is set at 50 percent.

In addition to reducing the top tax imposed on the income from savings and investment, this change also effectively reduces the maximum tax on long-term capital gains from 28 percent to 20 percent. Only 40 percent of a long-term capital gain is subject to tax.

By a special provision, this 20 percent ceiling is made retroactive to net long-term capital gains on transactions completed after June 9, 1981.

Of course if you're in, say, the 33 percent tax bracket, these provisions have no direct impact on your tax liability. They are intended to encourage a higher level of capital investment and savings by high-bracket people, who are generally the people who have substantial funds to invest.