Cation inter-diffusion between polycrystalline LaMnO3 and LaCoO3 pellets has been studied at 1373-1673 K in air by electron microprobe analysis. Inter-diffusion coefficients were evaluated by the Boltzman-Matano method from Mn3+ to Co3+ concentration profiles. The cation inter-diffusion is thermally activated and follows Arrhenius behaviour. The activation energies have been calculated and the mechanism for the B-site cation diffusion in La(Mn,Co)O3 solid solution suggested. Cation diffusion coefficients in the end members LaMnO3 and LaCoO3 were estimated. Cation mobility in LaMnO3 is higher than in LaCoO3. It is suggested that higher cation diffusion in LaMnO3 is due to the specific defect chemistry of this material, caused by the relative stability of manganese in a higher oxidation state (Mn4+). The results are compared to previous reports on cation diffusion in perovskite oxides.