The photo-oxidation method described by Armstrong, Williams and Strickland for determining total phosphorus and nitrogen in sea water has been adapted for analysis of fresh-water samples and extended to include the determination of total iron. The samples are subjected to ultraviolet irradiation in a photochemical reactor consisting of a high pressure mercuryarc lamp, a reactor body with twenty-four fused silica sample tubes and a cooling fan. Organically combined phosphorus and iron are converted into orthophosphate and iron(III) ions in the presence of acid and excess of oxygen after an irradiation time of 1 hour. Organonitrogen compounds and ammonia in solutions with pH range 6·5 to 9 are oxidised to nitrate and nitrite after 4 hours' irradiation in the presence of excess of oxygen, which excess is ensured by addition of a few drops of 30 per cent. hydrogen peroxide solution.