CHICAGO — The futures of two great American pastimes — consuming carbonated beverages and charging credit cards — are likely to be increasingly intertwined as vending machines that accept plastic become more prevalent.

It's a concept that inevitably raises questions about when a convenience might go from being a convenience to being an enabler, encouraging consumers to drink more soda, or buy more candy, while spending money they don't necessarily have.

Within the past week, MasterCard and a Coca-Cola bottling company have finished rolling out 1,000 cashless vending machines in the Philadelphia area so consumers with the urge to caffeinate can't be denied if they lack currency.

Their effort to make it easier for a tiny impulse purchase is part of a larger trend in which credit-card companies are doing their best to accommodate consumers who prefer to use credit, debit or prepaid cards for small-ticket purchases, even ones calling for only a quarter.

Such tiny transactions have even been tagged with a name: "micropayments," which Visa defines as $2 or less.

"If you need to pop a quarter in the parking meter, can you do that with your card instead?" said Niki Manby, senior vice president of product innovation for Visa. "We're looking at the whole gamut."

But the push toward encouraging electronic payments for vending purchases worries those who make a living encouraging people to be physically and fiscally fit.

"To me, that's scary," said Dave Grotto, a spokesman for the American Dietetic Association and president of Elmhurst, Ill.-based Nutrition Housecall LLC. "The best thing in life is not having spare change to add more calories to your life. Do I stop at the Snickers bar now that I can have a meal?"

Nancy Castleman, a consumer reporter for Cardratings.com, a Web site seeking to educate consumers about credit cards, also is wary of such machines.

"It may be really difficult for people who are already pumping more small change into vending machines than they should," Castleman said. "Snazzy machines selling stuff that is marketed to young people in particular are the last thing we need."

But a recent survey shows consumers are open to the idea of using plastic for vending machines. Nearly a third of respondents agreed with the statement, "I would use vending machines more if I could pay with a debit or credit card," according to the 80-page vending industry study released last December by market research firm Mintel International Group Ltd.

"There is unanimous agreement that card readers will become a prevalent feature in vending machines," said Elliot Maras, editor of trade magazine Automatic Merchandiser, "but the big question is when."

Although there have been bumps in the road in cashless vending, credit card companies believe the potential for increased card use in small-ticket segments is enormous.

About 60 percent of consumers carry less than $20 in cash, up from 49 percent just three years ago, MasterCard research shows.

Also, U.S. consumers make more than 350 billion transactions of less than $5 a year, representing $1.32 trillion in total spending, according to a 2004 survey by Ipsos-Insight, a market research firm.

In 2003, led by rapid growth of debit card payments, the volume of electronic payments surpassed check payments for the first time, according to the Federal Reserve Bank of Chicago.

By 2009 as many as a quarter of vending machines will accept electronic payments, estimates Michael Kasavana, National Automatic Merchandising Association endowed professor in the hospitality school at Michigan State University.