SummaryAqueous solutions of thiourea labelled with either 14C or 35S have been irradiated with γ-rays and high energy electrons. Chromatographic analysis of the irradiated solutions identified cyanamide, dicyandiamide and guanyl-thiourea; guanidine sulphate is probably another degradation product. These radiation products have been confirmed by the use of different solvents and sprays as well as by radioactive counting. Quantitative estimates of the yield of cyanamide have been made and differences between decomposition caused by light quanta and high energy radiation are discussed.