Biuret, if present in urea in high concentrations, has a poisonous effect on plants. For this reason, the mode of formation of biuret in the manufacture of urea has been investigated. The rate at which biuret is produced in molten urea and in concentrated aqueous urea solutions has been measured as a function of ammonia concentration. The relationship thus found can be explained by assuming that the direct conversion of urea into biuret and ammonia plays a secondary part, and that biuret is produced mainly by a reversible reaction between urea and isocyanic acid, the latter being formed from urea with simultaneous evolution of ammonia. The reversible character of this reaction is confirmed by the fact that the amount of biuret in a urea melt may decrease at high ammonia concentrations. Furthermore, the rate of conversion is increased by basic compounds and decreased by acidic compounds.