Following a discussion of the orthophosphato complexes of metal ions and systems involving quadrivalent vanadium, studies are described in which ion-exchange, pH-titration, spectrophotometric, and magnetic methods were employed to investigate the complexes of the vanadyl ion in aqueous media. The data obtained from studies of vanadyl/phosphate solutions under acid conditions have been interpreted as showing the occurrence of the following complex-forming reactions:- VO++ + H2PO4 VOHPO4 + H+ , VOHPO4 + H2PO4- HVO(HPO4)2-, HVO(HPO4)2- VO(HPO4)2-- + H+ , and parallel reactions involving phosphoric acid molecules as reactant species. The properties of a green solid, which precipitated from vanadyl/phosphate solutions of low hydrogen ion concentration, are reported together with a postulated polymeric structure for this material. The structure involves the linking of vanadyl ions by hydroxyl and phosphate bridging groups. Evidence is also reported which shows that species containing quadrivalent vanadium in alkaline phosphate media are of a different nature to those formed in perchlorate media. Studies of the vanadyl/thiocyanate system are described and the data obtained from these investigations are compared with those reported by other workers. Evidence is presented which shows that more than one thiocyanato vanadyl complex species co-exist in acid solutions, and that these are of low stability. The experimental data have been interpreted as showing that the species, VO(CNS)4--, is formed in significant concentration in the solutions. It is shown that the absorption spectra of vanadyl/thiocyanate solutions may be considered to indicate that the thiocyanato ligands are coordinated to the vanadium ions through their nitrogen atoms. Studies of the effects of hydrolysis on the absorption spectrum of the hydrated vanadyl ion are reported together with data which indicates that vanadyl ion forms complexes containing acetate, arsenate, bromide, chloride, oxalate and sulphate ligands.