U.S. consumers conducted nearly half of their online shopping using mobile devices, spending $5.9 billion in mobile commerce sales during the first quarter of 2013, according to Internet analytics firm Comscore.

Consumers using mobile devices accounted for 48% of total online shopping time, with 34% occurring on smartphones and 14% on tablets. The apparel and accessories segment was the highest-grossing retail mobile commerce category, accounting for nearly $1 billion in sales.

Retail e-commerce sales on desktop computers totaled $50.18 billion in the first quarter, excluding travel, auctions, autos and large corporate purchase transactions, according to Reston, Va.-based Comscore. The sales volume was up 13% year-over-year, but was down 11.6% from the 4Q12, when holiday shopping boosted online sales to $56.78 billion, the first time that U.S. e-commerce sales surpassed the $50 billion mark.

The rapidly expanding e-commerce sector is big business for the payments industry, where virtually all transactions are conducted electronically. In addition to providing payments acceptance and secure processing services for desktop and mobile e-commerce merchants, businesses like eBay’s PayPal unit are leveraging mobile to attract consumer adoption of their services, including more than 2.8 million new PayPal users added via mobile devices in the first quarter.

In desktop-based e-commerce, sales grew 20% or more year-over-year in five shopping segments: digital content and subscriptions; apparel and accessories; sport and fitness; consumer electronics and consumer packaged goods. Compared to the first quarter of 2012, the total number of e-commerce buyers increased 5% and average dollars spent per buyer was up 8%.

E-commerce sales volume has grown for 14 consecutive quarters, Comscore said, adding that online spending during the first quarter accounted for 10.6% of discretionary dollars spent — the highest share on record dating back to 2007.

“While the year-over-year growth rate of 13% remained healthy, it was a point or two below that of the preceding quarters. One potential explanation for this mild deceleration is the payroll tax increase, which went into effect in 2013 and which removed some disposal income from Americans’ wallets,” Comscore chairman Gian Fulgoni said in a press release. “That said, as long as job growth continues and consumer sentiment remains positive, the outlook for e-commerce in 2013 remains bright.”

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