Conductimetric titrations of the perchlorates [Pd(chel)2](ClO4)2 (chel = 2,2'-bipyridyl or 1,10-phenanthroline) with X- ions (X = Cl, Br, I) in the solvents nitrobenzene and nitromethane give an end-point after the addition of one equivalent of halide ion, owing to the formation of the ions [Pd(chel)2X]+. Another endpoint is obtained after the addition of a second equivalent of halide ion. Spectral evidence suggests that in nitrobenzene the compound [Pd(bipy)2I2] (bipy = 2,2'- bipyridyl) is formed. In nitromethane the second end-point is regarded as due to the reaction [Pd(chel)2X]+ + X- → [Pd(chel)X2]+ chel. The compound [Pt(bipy)2](ClO4)2 was prepared and titrated conductimetrically with halide ions in nitrobenzene and nitromethane. Only one end-point was obtained, indicating that the ions [Pt(bipy)2X]+, unlike the corresponding palladium complexes, are stable in the presence of excess halide ion. It is assumed that in all these monohalogeno-complex ions the metal atom is sexicovalent, due to the coordination of one molecule of solvent. Evidence is presented for the occurrence of six-coordinated platinum(II) in the compounds [Pt(bipy)2I(H2O)]I.H2O, [Pt(bipy)zI(H2O)](ClO4)2.H2O, and [Pt(bipy)2I2].