Four isomers of the complexes [Ru(CO)Cl₂(PMe₂Ph)₂L′][(I), (II), (IV), and (VI); L′= P(OMe)₃ or PPh(OMe)₂] can be isolated and characterized. Isomer (II) is obtained, directly or indirectly, by heating any of the otherthree isomers, but irradiation of (II) leads specificallyto isomer (I). A separate equilibrium exists between (I) and (VI). Isomers (I) and (II) of complexes [Ru(CO)X₂(PMe₂Ph)₃](X = Cl, Br, or I) can be similarly interconverted. Kinetic study of the rearrangement (I)→(II) for [Ru(CO)Cl₂(PMe₂Ph)₃] indicates that the initial step involves loss of a PMe₂Ph ligand: the five-co-ordinate intermediate obtained can either react directly with PMe₂Ph to form isomer (II) or rearrange prior to reaction with PMe₂Ph. Evidence of the lability of the bonds to both types of phosphorus ligand in the various isomers of the complexes [Ru(CO)Cl₂(PMe₂Ph)₂L′][L′= P(OMe)₃ or PPh(OMe)₂] suggests that mechanisms involving initial dissociation of a phosphorus ligand may also be involved in their interconversions.