Thermodynamic properties of Sr-doped perovskite-type lanthanum manganites (LSM) were generated by measuring the electromotive force (EMF) of rest materials, using the solid galvanic cell technique. The cell consisted of an anode (SrO/SrF2), an electrolyte (CaF2 or SrF2) and a cathode (LSM/SrF2). Two types of cells were tested in which type I used a CaF2 electrolyte and type II used a SrF2 electrolyte. The activity of SrO substituting into various LSM compositions was calculated using the measured EMF. The Sr content was varied from 0.16 to 0.6 mol, and the temperature of interest varied from 700 to 1000 degrees C. It was found that the SrO activity decreased with increasing temperature (at fixed composition) and increased with increasing Sr content (at fixed temperature).