In order to investigate the differences in the stability of the isomorphous pyridine saccharinates [Co(H2O)4(C5H5N)2](C7H4NO3S)2·H2O and [Ni(H2O)4(C5H5N)2[ (C7H4NO3S)2·4H2O, their thermal behavior (TG, DTG and DTA curves) from ambient temperature up to 1000°C in a static air atmosphere was studied. For comparative purposes, the thermoanalytical curves of [Cu(H2O)(C5H5N)2(C7H4NO3S)2] were recorded as well. The decomposition pathways and the stability of the compounds are interpreted in the terms of the structural data. A possible mechanism for the decomposition of the saccharinato ion/ligand in an oxidizing atmosphere is proposed. The infrared spectral characteristics of the complexes are also discussed.