The rates of mineralisation of urea and urea derivatives were studied in a laboratory anaerobic incubation experiment. Urea, urea phosphate and sulphur coated urea were hydrolysed rapidly and, even at the highest level of application, had disappeared in just over 8 days. The presence of the phosphate anion depressed pH in the early stages. Hydrolysis of the less soluble organic derivatives of urea, isobutylidine diurea, ureaform and glycoluril was very much slower and in the case of glycoluril a lag period of 8 to 16 days occurred before hydrolysis began.