The properties were studied of heated (320°C) chromia samples, prepared by two precipitation methods: 1. (1) addition of ammonia to chromium salt solutions, 2. (2) OH− formation in chromium salt solutions through hydrolysis of urea. Samples formed by means of the first method are macro or mesoporous and have a lower specific surface area (~200 m2·g−1) than those formed by urea hydrolysis (~300 m2·g−1). Only in the case of a very slow addition of the ammonia solution these properties of the chromia's become equal. Experiments show that the micro porous type samples with high surface area are only formed if the pH range 5.1 to 5.7 is passed slowly. The formation of polychromium complexes of uniform size is suggested.