Apple's new OS X, dubbed Mountain Lion, will exclude some older Macs that can run 2011's Lion, the company's website said.
The system requirements for OS X 10.8, slated to ship this month, ban older machines including the plastic-encased iMacs introduced in January 2006, MacBooks prior to the first all-aluminum model rolled out in late 2008, MacBook Pros older than those introduced in June 2007, and the first-generation MacBook Air, which debuted in January 2008.
Apple's list of no-Mountain-Lion-for-you now on its site is identical to the one it introduced last February when it started distributing OS X previews to developers.
Mountain Lion's requirements are more stringent than its predecessor, OS X 10.7, or Lion, which only demanded that the machine have 2GB of memory and an Intel Core 2 Duo, Core i3, Core i5, Core i7 or Xeon processor.
But that won't cut it this time.
For example, some Core 2 Duo-powered Macs can run Mountain Lion, but others cannot.
The original MacBook Air came with 2GB of RAM and used a Core 2 Duo CPU, so it could run Lion. But it's not on the upgrade list. The same goes for 15- and 17-in. MacBook Pros launched in October 2006, which were also equipped with a Core 2 Duo.
Apple did not immediately reply to questions, including what precluded older machines from running Mountain Lion even though they could handle its ancestor.
Microsoft, meanwhile, has said that PCs now running Windows XP -- which is nearly 11 years old -- Vista or Windows 7 can be upgraded to Windows 8 later this year as long as they meet very lenient hardware requirements that include 1GB of RAM and a 1GHz or faster Intel or AMD processor.
Mountain Lion, which will sell for $19.99 through the Mac App Store, will ship this month, Apple has said. Based on the company's past practice, Computerworld has pegged the likely ship date as Wednesday, July 25.
On Monday, Apple shipped a "golden master" of Mountain Lion to OS X developers, signifying that the code is completed, or nearly so.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is firstname.lastname@example.org.
This story, "Some Lion-Powered Macs Won't be Upgradable to Mountain Lion" was originally published by Computerworld.