Abstract Pseudotetrahedral [CoX2P2] and [CoX3P]− complexes (X = Cl, Br, I; P = tertiary phosphine) undergo practically complete solvolytic displacement of phosphine in coordinating solvents such as acetonitrile, where phosphine-containing species can be formed only partially even in the presence of large excess of ligand. [CoX2P2] and [CoX3P]− are more stable in non-coordinating solvents like dichloromethane, but limited solubility and solvent-ligand interactions still make accurate measurements of the phosphorus-containing chromophores difficult. Values of equilibrium constants are reported for the reaction [CoX3P]− + X−⇄ [CoX4]2− + P in CH2Cl2; phosphines replace halides with increasing facility in the order Cl PPh3.