Thousands of upper-income taxpayers would face effective top marginal income tax rates considerably higher than the prominently featured 27 percent rate in the tax-revision bill approved this week by the Senate Finance Committee.

In some very rare instances, taxpayers could have a top rate higher than the 50 percent rate in current law. A greater number would pay at effective top rates higher than the 38 percent rate in the House version of the bill.

The higher-than-advertised rates arise from several provisions included in the Senate bill to limit the amount of the tax cut for upper-bracket taxpayers.

Here is what happens:

Taxpayers would continue to make certain adjustments to come up with a figure for adjusted gross income. From it, they would subtract either a standard deduction -- $5,000 on a joint return, $3,000 for a single individual -- or a larger amount for itemized deductions. Then they would subtract a $2,000 personal exemption each for the taxpayer or taxpayers and their dependents.

The amount left is taxable income. The first $29,300 of taxable income on joint returns and the first $17,600 on a single return would be taxed at a 15 percent rate. All taxable income above that level supposedly would be taxed at a 27 percent rate.

But once taxable income reaches $75,000 on a joint return, part of that first $29,300 that was taxed at the 15 percent rate begins instead to be taxed at the higher 27 percent rate. As income rises from $75,000 to $145,320, each additional $2.40 of income moves $1 from the 15 percent bracket to the 27 percent bracket.

This phase-out of the lower bracket is the equivalent of adding 5 percentage points to the tax rate on income between $75,000 and $145,320, making the effective marginal rate on that income 32 percent rather than 27 percent. For single taxpayers, the phase-out runs from $45,000 to $87,240.

Once those income levels are exceeded, the personal exemptions begin to be phased out, and are gone entirely when income reaches $185,320 on joint returns and $127,240 on single returns. How much this adds to the effective top tax rate depends on how many personal exemptions are claimed on the return. Each one adds 1.35 percentage points to the rate.

Thus, a couple with no dependents would be paying a rate 2.7 percentage points higher, or 29.7 percent. If six exemptions were claimed, the rate would be 35.1 percent.

There is a final provision involving a phase-out of an exception limiting the deductibility of certain real estate and other losses. As the income of the small number of affected taxpayers rises from $100,000 to $150,000, the provision would make whatever marginal rate was otherwise being paid half again as high. In other words, for most of this group, the rate would rise from 32 percent to 48 percent.

Finally, a large share of income received by upper-bracket taxpayers usually comes from long-term capital gains, which currently are taxed at a top rate of 20 percent. The Senate bill would treat capital gains like other income, and it is not yet clear whether gains would carry a new top rate of 27 percent, or whatever higher effective rate a taxpayer might be paying.

Once all the phase-outs are completed, the effective top marginal rate falls to 27 percent and remains there.