Effect of gamma -irradiation (410-MGy) and the presence of DY2O3 (10 mol%) on the decomposition of anhydrous barium oxalate has been investigated by the rising temperature technique. It is evident from the data that in all the materials, decomposition occurs through (i) a slow reaction stage and (ii) a fast stage followed by a decay period. The loss of 'CO' from the oxalate is governed by a two-dimensional phase boundary reaction with cylindrical geometry, the power law of the reaction order, 0.5, in the irradiated salt and three-dimensional nuclei growth in case of mixture. The data were well explained by the contracting area (R-2) model indicating that irradiation as well as mixing enhanced the rate of reaction and decreased the energy of activation, the effect being more prominent in the latter case.