The tax bill approved by the House of Representatives approved in late December injects new uncertainties into the tax costs of some corporate mergers.

Tax experts said it is difficult to determine whether the restrictions will kill deals that otherwise would have been made. "You never know the deals that might have been," said one.

But the House bill's provisions on how an acquiring corporation accounts for the value of the assets it acquires in certain types of takeovers will make buyers more cautious about how much they are willing to pay, experts said.

In the House version of tax revision, a corporation that acquires assets in some takeovers would be penalized for boosting for tax purposes the value of the acquired assets to reflect the difference between what the acquiring company pays and the value of the assets on the books of the acquired company.

If the company can increase the value of the acquired assets -- buildings, inventories and the like -- it can increase the depreciation deductions it takes and increase the tax-deductible costs of inventories. Both actions reduce the taxes the acquiring company must pay -- without having any real impact on its cash flow.

The House version of tax revision restricts or eliminates the ability of acquiring companies in some types of takeovers to "step up" the value of the acquired assets. The effective date of the change is Nov. 20, 1985.

The changes would increase the cost of the acquisition to the purchasing company and reduce the "premium" it is willing to pay to the company whose assets are being acquired.

"The problem is that we don't know the effective date," said tax lawyer Leonard Silverstein. At present, he said, it must be assumed that the effective date will not change, even though the final law, if it is passed, could change either the effective date or the House provisions themselves. But purchasing companies will have to take steps to protect themselves in case they have to pay retroactive higher taxes, experts said.

"We are in a state of permanent uncertainty," according to David A. Berenson, national director of tax policy and practice for the big accounting firm Ernst & Whinney. "American business can live with bad tax law. But it can't live with uncertain tax law."

So far it is hard to determine whether any deals have been canceled because of the changes.

Several reports last month said the potential changes in the tax law convinced potential purchasers of the Outlet Broadcasting Corp. to pull out of a deal last month. But according to Outlet executives -- who have found another group to assist them in buying the company from its parent Rockefeller Group -- the original deal was killed for non-tax reasons.