Abstract: Yellow iron oxide pigments having improved heat stability are prepared by subjecting an alkaline slurry of conventional yellow iron oxide, a soluble ferric salt, and sodium aluminate to a hydrothermal (autoclave) treatment at 100.degree.-240.degree. C., separating and slurrying the resultant solid product, and subjecting that product to a second hydrothermal treatment of 250.degree.-350.degree. C. The original iron oxide has a heat stability (based on color change from yellow to red) temperature of about 203.degree. C., while the final product shows a change at 270.degree. C.