Diazoacetic esters reacted with 4-substituted 1,2,4-triazole-3,5(4H) -diones (2) to give one-to-one adducts. Molecular weight determination, high-performance liquid chromatography, and 2D n.m.r. spectroscopy showed that the adducts are monomers that associate in solution. The monomers are most likely 1,3,5-triazabicyclo[3.1 .O] hexane-2,4-diones. The adducts were thermally isomerized to 3,6disubstituted 1,3,5-triazine-2,4( 1 H,3H) -diones. Diphenyldiazomethane reacted with (2) to yield an azomethine imine, which underwent thermal dimerization followed by rearrangement.