To study the superexchange interactions in 'giant magnetoresistors'-manganites with perovskite structure, diluted solid solutions of La0.67Ca0.33MnO3-LaAlO3 were prepared on the basis of MnO2 and Mn2O3. Examination of temperature and concentration dependencies of paramagnetic susceptibility and effective magnetic moment showed a strong tendency of manganese atoms to clustering in the solutions. The clusters are linked by ferromagnetic superexchange with an exchange parameter varying with temperature, which correlates with the unusual electrophysical properties of pure oxides and can be accounted for by varying the superexchange angles Mn-O-Mn with temperature.