The catalytic activity of rare earth oxides (REOs) on the isothermal decomposition of calcium oxalate has been studied within the temperature range 693-733 K. The fractional decomposition, alpha, versus time, t, plots for pure salt as well as for mixtures suggest that the process occurs through initial gas evolution, acceleratory and decay stages. Analysing the data in the light of theories of various kinetic models, it is indicated that the two-dimensional phase boundary reaction model gives the best fit of the results. Admixtures enhance the rate of reaction and decrease the energy of activation of both acceleratory and decay stages, the catalytic activity being in the order : La2O3 > Sm2O3 > Gd2O3. A plausible mechanism of decomposition and the role of the catalyst in the process have been discussed.