A method is described for the determination of thiosulphate in urine. After removal of interfering compounds, including endogenous thiocyanate by ion exchange, thiosulphate is converted to thiocyanate in the presence of cyanide and cupric ions. The thiocyanate formed is concentrated by ion exchange, eluted with an acid solution of ferric ions and the ferric thiocyanate complex determined colorimetrically. Healthy human subjects excreted 31.7 ±12.8 μmol/24 h (mean ± SD) thiosulphate.