Barium(II)bis(oxalato)barium(II)dihydrate (BOD), Ba[Ba(C2O4)(2)]. 2H(2)O and lead(II)bis (oxalato)lead(II)monohydrate (LOM), Pb[Pb(C2O4)2]. H2O have been synthesized and characterized by elemental analysis, conductance measurements, IR spectral, reflectance and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air showed that at ca. 1000 degrees C, a mixture of BaO2 and BaCO3 is generated from the compound, BOD, through the formation of BaO2 and BaC2O4 at around 514 degrees C as intermediates. LOM gave Pb2O3 as final product at ca. 390 degrees C. DSC study in nitrogen showed a different decomposition pattern from that in air for both compounds. Using seven mechanistic equations, the rate controlling process of the dehydration and decomposition mechanism of BOD is inferred to be one- and three-dimensional diffusion, respectively. The decomposition mechanism of LOM is a phase boundary reaction having cylindrical symmetry. Some of the decomposition products were identified by analytical, IR spectral and X-ray powder diffraction studies. A tentative reaction mechanism for the thermal decomposition of both the complexes is proposed.