Proton and 31P n.m.r. have been employed to investigate the constitution and behaviour of zinc(II)O,O′-dialkyl dithiophosphates in solution. The complexes Zn[S2P(OR)2]2(R = Et or Pri) are principally dimeric in chloroform and toluene with dissociation constants, KD(R, solvent), to monomers of 1.81 × 10–2(Et, CDCl3), 3.08 × 10–2(Et, toluene), 4.45 × 10–2(Pri, CHCl3), and 7.17 × 10–2 mol I–1(Pri, toluene) at 29 °C. Higher 31P chemical shift values and conductance data for solutions of these compounds in ethanol and tetrahydrofuran indicate that co-ordination of solvent and ligand ionization occur in these solvents. The 31P chemical shifts for complexes Zn[S2P(OR)2]2·L increase in the order L = pyridine < 2,2′-bipyridine < 2,2′: 6′,2″-terpyridine, and comparison with known structural data has shown that this change is due primarily to an opening of the SPS bond angle of the S2P(OR)2 ligands. The ‘basic’ zinc(II)O,O′-dialkyl dithiophosphates, Zn4[S2P(OR)2]6O, are unstable in solution, spontaneously, though reversibly, decomposing to ‘normal’ Zn[S2P(OR)2]2 and zinc(II)oxide as the temperature is increased. Pyridine (py) causes an irreversible decomposition of Zn4[S2P(OR)2]6O to Zn[S2P(OR)2]2·py and zinc(II).