Reaction kinetics between dense, polycrystalline pellets of La2O3 and CoO were investigated at temperatures of 1370-1673 K and oxygen partial pressures of 40 Pa - 50 kPa. At high oxygen partial pressures, single phase LaCoO3 was formed. The growth of the LaCoO3 phase followed the parabolic rate law. The location of Pt markers demonstrated that diffusion of Co3+ cations in LaCoO3 dominated over diffusion of La3+. The diffusion coefficient of Co3+ was determined from the parabolic rate constant, and an activation energy of (250 +/- 10) kJ mol-1 was found. The diffusion coefficient of Co3+ in LaCoO3 decreased with decreasing oxygen partial pressure. At the lowest oxygen partial pressure investigated, two product phases, LaCoO3 and La2CoO4, were observed. The diffusion coefficient of Co cations in La2CoO4 was estimated. Results were discussed in relation to cation diffusion in other LnBO3 oxides (B = Cr3+, Mn3+, Fe3+). A correlation between diffusion of the B cation and the melting point was found for LnBO3 materials.