Continuous solid solutions with a tetragonal structure of K2NiF4-type are formed in the SrLaGaO4- SrLaAlO4 system. Samples of SrLaAlxGa1-xO4 (0.0 less than or equal to x less than or equal to 1.0) were obtained by the freeze-drying method and their structural parameters and thermodynamic properties were studied by X-ray diffraction and heat flux Calvet calorimetry. Composition dependence of lattice constants was observed to follow Vegard's low. Heats of solution of the SrLaAlxGa1-xO4 samples in molten 2PbO .B2O3 were measured, and enthalpies of formation and mixing were calculated. The possibility of eliminating the lattice mismatch of these phases and YBa2Cu3Ox (x = 6.90-7.00) or Bi2Sr2CaCu2O8 by proper selection of the Al/Ga ratio makes them perspective substrate materials for high-temperature superconducting thin films.