In this thesis an attempt has been made to further the knowledge of aqueous complexes of tervalent gallium and indium phosphates and to examine ion exchange and pH titration methods for their investigation. A general treatment is presented for the interpretation of results of pH titrations in which phosphoric acid is added to a metal salt solution. Conditions producing variable results have been considered and suggestions are made for the standardisation of such titrations. Sorption of gallium and indium on a cation exchange resin at different pH values has been studied and explanations have been presented for the variation, with pH, in the quantity of metal sorbed. The ageing of solutions of gallium and indium chloride has been found to have an important effect on the subsequent reaction when phosphoric acid is added to the solution. This has been shown both by ion exchange and pH titration methods. Ion exchange results have indicated the presence of both anionic and cationic complex phosphates of gallium and possible structures for these complexes have been put forward.