The thermal decomposition of solid complexes of palladium(II) chloride with pyridine and several methyl and dimethylpyridines has been studied using differential scanning calorimetry. In general, a linear relationship exists between the p K b values of the ligands and the activation energies for decomposition of the complexes. The variation of initiation temperatures for the decomposition with ligand basicities is interpreted in terms of a possible trans effect.