Potassium tris(oxalato)lanthanum(III)nonahydrate, K-3[La(III)(C2O4)(3)].9H(2)O, has been synthesized and characterized by elemental analysis and IR spectral studies. Thermal studies (TG, DTG and DTA) in air showed that the anhydrous compound formed at ca. 192 degrees C. The final end product at 1000 degrees C was shown to be a mixture of La2O3, K2CO3. K2O, KLaO2 and La2C3, through the formation of an intermediate mixture of K2C2O4 and K[La(C2O4)(x)] (where 1.5 < x < 2) at ca. 293 degrees C, and K2C2O4 and La2O2CO3 at ca. 528 degrees C. DSC study in nitrogen upto 650 degrees C showed that the end product was a mixture of K2C2O4 and La2O2CO3. The kinetic parameters, mainly E* of the dehydration and decomposition steps in TG, were calculated using four non-mechanistic equations. The kinetic parameters, E*, Delta H, Delta S obtained from DSC study are discussed. Some of the decomposition products were identified by IR and X-ray powder diffraction studies. A tentative mechanism for the decomposition in air is proposed.