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Walker & Hambly (1895) showed that the isomeric transformation of ammonium cyanate into urea (Wohler, 1828) was spontaneously reversible in aqueous solutions. At equilibrium at 1000, a 0.1M-solution of urea was said to contain approximately 95% urea and 5% ammonium cyanate. Werner (1923) denied formation of cyanate from urea at temperatures below 60°. According to this author aqueous solutions of urea, kept for many months at room or body temperature under sterile conditions, did not contain traces of cyanate. Since some pharmacological actions of cyanate were recently described (Schutz, 1945, 1946; Birch & Schutz, 1946), the question whether cyaneFte is formed in the mammalian organism was investigated. As the first step in studying this question, experiments were carried out with the sole aim of establishing whether cyanate was spontaneously formed inaqueous solutions of urea, at body or room temperature. Experiments dealing with living tissues will be reported separately. The isomeric change in question could, theoretically, be demonstrated by showing a gradual decrease in the amount of urea originally present in-the solution. Since, however, even at high temperatures, only a small percentage of the urea originally present is known to undergo the isomeric change into NH4CNO, the expected decrease would probably be within the margin of error of the most sensitive quantitative methods for the determination of urea. On the other hand, the existing methods for the detection and quantitative determination of cyanate (Walker & Hambly, 1895; Hertig, 1901) were found to be too insensitive, especially when urea was present in higher concentrations. In this paper two new methods are described for the qualitative and quantitative determination of cyanate in aqueous solutions. By means of these procedures evidence was obtained for the spontaneous formation of cyanate in aqueous solutions of urea, at body and room temperature.