The next time you pay for gasoline at the pump, you may be using more of your credit line than you bargained for.

It's routine for car-rental companies and hotels to block off a portion of the credit limit on your credit card when you rent a car or begin a hotel stay. Understandably, they want to be sure there's enough left to cover your final bill.But drivers who pay for gasoline by using their credit cards at automated pumps may be surprised to discover similar holds on their credit lines.

Crown, Exxon and Sunoco are among the retailers that kidnap your credit by locking up $35 to $50 when you buy gas on certain cards, no matter how much you actually purchase.

A string of $5-to-$10 gas purchases within a week or two could lock up hundreds of dollars and even cause your card to be rejected.

Credit-card companies say they replace the "authorization" charge with your actual gas charge within two to eight days. But smaller and less-efficient banks require almost an entire month to do this. Oil companies say the authorizations are necessary to protect themselves against fraud.

When you run your card through the magnetic reader at the pump, it is connected by phone line to the card issuer's authorization system only long enough to confirm that the card isn't stolen or over the limit.

Industry executives say it's impractical to leave the line open the entire time you're pumping gas, so the companies "hold" $35 to $50 of your credit line until the actual charge comes through.

Off the record, oil-company executives admit gas stations have not kept pace with the point-of-sale technology now available. Eventually gas stations will upgrade their pumps and be able to transmit the exact charge to the card issuer.

In the meantime, consumer complaints have prompted some gas companies to change their authorization practices. Mobil and Shell credit cards now block off only $1 of your credit line, while allowing you to pump up to $36 or $50 worth of gas.

The same policy applies when you use a Visa card at those stations.