Windows lost nearly a full percentage point of market share for the second month in a row in December, pushing Microsoft Corp.'s operating system to a new low, an Internet measurement company reported yesterday.

Meanwhile, Apple Inc.'s Mac OS X posted a record gain that brought it close to a 10% share for the first time since Net Applications Inc. began tracking operating system use.

In December, 88.7% of the people who browsed the Web sites that Net Applications monitors did so using machines powered by Windows, a 0.94 percentage point drop from November. The slide was Windows' largest in the four years that Net Applications has collected operating system data, and the second record-setting monthly loss in a row for Microsoft's software.

During November, Windows' market share slipped 0.84 percentage point, dropping the operating system under 90% for the first time since Net Applications has been tracking operating system use.

The combined decline of November and December totaled 1.8 percentage points, Windows' biggest two-month dip ever, nearly double that of its previous record, a 0.92 percentage point fall in December 2007 and January 2008.

Microsoft's operating system ended the year down 3.1 percentage points, a 3.4% drop in its share from the same time last year.

Meanwhile, Apple's Mac OS X's market share continued to grow at Windows' expense. For the second month in a row, Mac OS X posted a record increase, growing by 0.76 of a percentage point to end the month at 9.6%. December was the first time that Net Applications had pegged Apple's operating system above the 9% mark.

And just as Windows set a record for a two-month drop, Mac OS X set a record for a two-month increase during November and December. Those months' combined gain of 1.4 percentage points was substantially larger than the earlier record, a 0.9 percentage point boost the operating system received in September-October 2006, and almost double the 0.73 percentage point increase of November-December 2007.

Mac OS X's market share was up 2.3 percentage points during 2008, an annual growth rate of 31.7%.

Net Applications again attributed some of the decline of Windows and the corresponding growth of Mac OS X to special circumstances. "The December holiday season strongly favored residential over business usage," the company said on its Web site. "This in turn increases the relative usage share of Mac ... and other products that have relatively high-residential usage. All December usage statistics should be read in that context."