Members of the system Cd 2Ge 1- xSi xO 4 where 0 ≤ x ≤ 0.4 have been prepared. These compounds were observed to crystallize with the olivine structure, space group Pbnm. The resistivity, Hall mobility, flat-band potential, band gaps, and stability were determined as functions of composition. The variation of these photoelectronic properties can be attributed to the reduction of the cell parameters with increasing silicon substitution. The substitution of silicon for germanium reduces the loss of photocurrent from 25% after 1 hr for x = 0.0 to only 6% after 22 hr for x = 0.4.