The isomerization of ammonium cyanate in dimethyl sulphoxide leads to urea with a substantial side reaction yielding biuret (c. 25 % at 60°). Biuret is formed in a process that is competitive with urea formation and not in a subsequent reaction involving urea. The production of urea, as in water, is by the reaction of ammonia and cyanic acid molecules and it is postulated that biuret is formed through an intermediate of dimerized cyanic acid. Temperature and ionic strength dependence have been studied. Kinetic studies of the isomerization of methylammonium and ethylammonium cyanates are also described.