Abstract Some solid palladium(II) and platinum(II) complexes of ethylthiourea (Etu) were prepared. Those with stoichiometry M(Etu) 4A 2 (AClO 4, BF 4, CF 3COO) and M(Etu) 4X 2 (XCl, Br, I) behave in solution as 1:2 electrolytes; the complexes MEtuX 2 (XCl, Br, I) are non-electrolytes, presumably with a dimeric halogen-bridged structure. High-frequency titrations indicated the formation of halide complexes with a ratio of Etu:M of 1:1, 2:1 and 4:1 in acidic, aqueous and dimethylformamide (DMF) solutions, respectively; Pd gives also the 3:1 complexes in DMF. By paper chromatographic elution of mixtures of the complexes and 35S-labelled ethylthiourea with aqueous solutions (0.5–5.0 M) of the corresponding NaA or NaX salts, the migrating species M(Etu *) 4X 2 and M(Etu *) 4A 2 were observed. In the case of the chlorides and bromides, two complex species migrating on the chromatographic paper in compact and distinct spots were observed: one with an R F value almost equal to that of the M(Etu *) 4 2+ species, the other with a much lower R F value. With increasing NaX concentrations of the eluting solution, the first complex decreases until it disappears and the second increases up to 100% for solutions 4–5 M in NaX. The second species can be considered as a square pyramidal five-coordinated complex, [ML 4X] +.