Continuous solid solutions with a tetragonal structure of K 2 NiF 4 -type are formed in the SrLaGaO 4 -SrLaAlO 4 system. Samples of SrLaAl x Ga 1 - x O 4 (0.0 =< x =< 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 SrLaAl x Ga 1 - x O 4 samples in molten 2PbO.B 2 O 3 were measured, and enthalpies of formation and mixing were calculated. The possibility of eliminating the lattice mismatch of these phases and YBa 2 Cu 3 O x (x = 6.90-7.00) or Bi 2 Sr 2 CaCu 2 O 8 by proper selection of the Al/Ga ratio makes them perspective substrate materials for high-temperature superconducting thin films.