Consumers in the United States spent $53.3 billion with e-retailers during the third quarter, up 14.6% from $46.5 billion during the same quarter last year, according to web measurement firm comScore Inc.

But e-retailers may be noticing a change as to how consumers are placing their orders. ComScore says $5.8 billionof the $53.3 billion in sales recorded in Q3 took place on mobile devices, which translates to 10.9% of all web sales during the quarter. That’s up from 9.9% a year ago, when mobile accounted for approximately $4.6 billion in web sales and Q3 web sales totaled $46.5 billion.

Smartphones accounted for 62% of mobile web sales, and tablets for the remaining 38%, comScore says. Mobile retail commerce in the United States is projected to grow 63% this year to around $34.17 billion from $20.95 billion in 2012, according to Internet Retailer’s 2014 Mobile 500, which ranks online retailers by their 2013 mobile commerce sales. The 2013 growth projection includes an estimated $8.8 billion in U.S. merchandise sales taking place on eBay Inc. 

Looking only at sales taking place on desktop computers, comScore says retail e-commerce sales grew 13.4% year over year during Q3, to $47.5 billion in 2013 from $41.9 billion a year ago. “Third quarter [desktop] e-commerce spending grew 13% from a year ago, and although that marks a pretty healthy growth rate, it also represents a slight deceleration from the prior quarter,” says Gian Fulgoni, comScore chairman. ComScore recorded $49.8 billion in desktop web sales in Q2 2013, meaning consumer spending from desktop computers dropped about 4.6% quarter over quarter. In 2012, there was a 2.8% decline in online sales in the third quarter compared with the second quarter. Fulgoni says this suggests a softness in discretionary spending going into the holiday season. “Nonetheless, we are confident that the growth rate in online spending will once again far exceed that in bricks-and-mortar stores, reflecting the ongoing channel shift to e-commerce.”

For its desktop e-retail estimates comScore draws on online purchase data from its panel of about 1 million U.S. online shoppers and excludes automobile and auction sales. For mobile commerce estimates, comScore says it surveys Internet users about their spending by platform across product categories and calibrates those numbers according to behavioral data it has on desktop e-commerce. Like desktop estimates, mobile commerce estimates also exclude automobile and auction sales.

The U.S. Department of Commerce releases its third quarter e-commerce estimates Nov. 22. Read more about how mobile commerce is blasting off for web retailers in Internet Retailer Magazine’s October cover story.