Solid solubility and structural phase transitions in (Ca x Sr1-x )TiOGeO4have been studied by means of in situ high temperature X-ray powder diffraction. The displacive A2/a–P21/a phase transition analogous to titanite has been followed across the solid solution. Strain analysis indicates a transition temperature of T c=594 ± 10 K for SrTiOGeO4 and the additional occurrence of an isosymmetric anomaly at T i =800 ± 25 K, in analogy to the isomorphous compound CaTiOGeO4. Lattice parameters as a function of temperature and composition have been determined by X-ray powder diffraction between room temperature and a maximum temperature of 1123 K. The e 11 and e 13 components dominate the strain tensor. All compositions across the solid solution exhibit close to tricritical phase transitions P21/a–A2/a. The critical temperature remains almost unaffected by substitution of Sr for Ca, but the magnitude of the spontaneous strain drops significantly with even small amounts of Sr present.