mugsworth, All this information might provide some usefulness if the issue was about the RAM requirement to process a particular image size. But the following issue remains unsolved. Available RAM = 954625 Used by Photoshop = 75% Photoshop RAM (available to photoshop) = 715968 Disk space (cache) available to Photoshop = 38200000 (The k to mg conversion you reference creates a differential so minimal it need not be considered.) Size of image requiring RAM or disk cache to process = 0 Size of Photoshop pstemp file = 48960 Your last comment, "In simple terms, placing a temp file on the disk when loading means that the available RAM PhotoShop is set to use; is simply not there". Obviously this rule is contrary to how the program actually behaves. I think the ultimate answer to this is that it is simply a quirk of Photoshop. Thanks for all your kind efforts and advice. Pinto P.S. What is your available RAM and how big is your pstemp file when Photoshop loads?