Online spending in March increased 16.1% year over year, the fifth straight month of double-digit growth for e-commerce, according to an estimate released today by MasterCard Advisors, the consulting arm of MasterCard. E-commerce spending increased 13.2% year over year in February.

MasterCard bases the SpendingPulse report on retail sales using all payment forms, including credit and debit cards of all brands, cash and checks. This month’s report did not include actual spending in dollars.

Online apparel sales also posted a large gain last month, increasing 18.7% year over year, the 16^th consecutive month of growth for the sector. By contrast, total apparel sales, which includes purchases made in bricks-and-mortar stores, increased 4.4% in March, the payment card network says. This month’s report offered no more e-commerce figures, but did note that overall luxury sales had its sixth consecutive month of growth, up 8.5% year over year, the largest monthly gain since December.

“Most retail sectors continued to record solid growth year-over-year, similar to February, although we’re not seeing an acceleration of momentum from February to March,” says Michael McNamara, vice president, research and analysis, for MasterCard Advisors SpendingPulse. “The lack of increased momentum in some sectors could be due to calendar shifts, given that Easter falls very late this year.”  

High gasoline prices, he adds, could dissuade consumers from making trips to bricks-and-mortar stores, dampening retail growth. “On the other hand, we’ve seen the e-commerce channel benefitting somewhat from this trend,” he says.

The January gain in online sales was the second-highest in the recent five-month surge. December 2010 e-commerce sales grew 17.6% over December 2009, MasterCard says.