Thermal cyclization of the pyrimidine- N-oxide dicarbamate 2 gives a 95:5 mixture of the 2-oxo-2 H-[1,2,4]oxadiazolo[2,3- a]- and [2,3- c]pyrimidinecarbamates 3 and 4. An efficient procedure for the conversion of 3 to 4, and vice versa, is described.