Recovery of guanidine carbonate from aqueous solutions

In a process for isolating guanidine carbonate from a dilute aqueous solution which contains guanidine, urea and pyrolysis products thereof, ammonia and carbon dioxide, the improvement which comprises evaporating the solution at a temperature from 80.degree. to 130.degree. C until a content of guanidine carbonate of at least 20 g/l and a content of pyrolysis products of not more than 28 g/l at evaporation temperature of 80.degree. C, or not more than 128 g/l, at on evaporation temperature of 130.degree. C, is achieved, whithout exceeding a heating time of 6 hours at the upper limiting temperature, filtering off precipitated pyrolysis products, subsequently cooling the resulting solution to 45.degree. C and below and adjusting the ammonia content of the solution to 15 to 30 % by weight by adding gaseous or liquid ammonia or a concentrated aqueous solution of ammonia and separating the resulting precipitated carbonate.