Several compositions in the system Sr(1.45-x)CaxSb0.55O3-y (0 less than or equal to x less than or equal to 1.45) have been synthesized by solid state reaction in air at 1000 degreesC and investigated by powder X-ray diffraction and EDS microanalysis. They exhibit good thermal stability up to 1200 degreesC. The solid solution range was delimited in the composition 0 less than or equal to x less than or equal to 1.1, For quantities of Ca2+ higher than 1.1, the amount of solid solution decreases and another phase, Ca6Sb2O10, appears. The lattice parameters of the system Sr(1.45-x)CaxSb0.55O3-y decrease with increasing Ca2+ content.