AbstractThe activation energy parameters for the reaction of PdX 4 2−(X=Cl−, Br−) in aqueous halide acid solution with thiourea (tu) and selenourea (seu) have been determined. High rates of reaction parallel low enthalpies and appreciable negative entropy of activation. The rate law in each case simplifies tokobs=k[L] where L=tu or seu, and only ligand‐dependent rate constants are observed at 25°C. The ligand‐dependent rate constants for the first identifiable step in the PdCl 4 2−+ X system is (9.1±0.1) × 103M−1sec−1and (4.5±0.1) × 104M−1sec−1for X=tu and seu, respectively, while for the PdBr 4 2−+ X system it is (2.0±0.1) × 104M−1sec−1and (9.0±0.1