The effect of silica concentration on the solubility of A1 and Ti in diopsidic pyroxenes has been investigated at one atmosphere in the system diopside-CaTiA1206-SiO 2 at temperatures between 1150-1420 ~ C. The composition of pyroxene in the system diopside-CaTiA1206-Si02 is influenced by the total Si02 content. Near the join diopside-CaTiAleO6, the pyroxene forms a solid solution with the CaTiAl20 s molecule, and co-exists with perovskite for compositions greater than 11 weight percent CaTiA1206. Anorthite is an accompanying phase. With increasing total SiO 2 content a series of mineralogical changes involving Ti-bearing phases occur. Sphene solid solution co-exists with diopside solid solution, anorthite, and perovskite in a small compositional range near the diopside-CaTiA1206 join. Additional total SiO 2 results in the elimination of perovskite and a decrease of solid solution in the pyroxenes. With further increase in SiO 2 content, tridymite appears and the pyroxene is approximately pure diopside. Rutile joins diopside, anorthite, sphene solid solution and tridymite over a broad compositional range in the Ti, Si-rich part of the system. These results demonstrate that increased silica concentration decreases the solubility of A1 and Ti in diopsidic pyroxenes and controls the stability of co-existing Ti-bearing phases.