In order to investigate the role of Ca content on the nature of the active phase of silica supported molybdenum based catalysts, we have prepared by a two-step procedure, a series of catalysts containing the same amount of molybdenum (1, 2, and 10 Mo wt%) but with variable Ca/Mo atomic ratios, ranging from 0 to 4. We have shown by XRD, Raman spectroscopy, and methanol oxidation reaction, that molybdenum species react with previously deposited Ca, leading to the formation, for CalMo atomic ratios lower than one, of two kinds of calcium molybdate species on the silica support : (0) crystallized CaMoO4 particles originating from calcium phase large clusters; (b) highly dispersed CaMoOx species formed from highly dispersed Can ions. These latter species have been found catalytically very active in methanol oxidation for the selective formation of methyl formate, whereas supported CaMoO4 particles behave as bulk CaMoO4 (low catalytic activity and carbon oxide formation), as observed for CalMo atomic ratios higher than one, where only crystallized CaMoO4 particles are formed.