The potential energy hypersurfaces corresponding to the addition reactions of fulminic acid (HCNO) to water, ammonia, and methanol have been explored using ab initio molecular orbital calculations. Geometries of stationary points were optimized at the HF/3-21G, HF/6-31G*, and MP2/6-31G* levels of theory while relative energies were estimated at the MP4-SDQ/6-31G* and MP2/6-31G** levels. Each addition is predicted to be concerted with a single transition structure connecting the reactants and the oxime adduct. This confirms the result of an earlier ab initio study (J. Am. Chem. Soc. 1980, 102, 573) and points out a difference with recent MNDO results (J. Mol. Struct. Theochem 1988, 181, 389), which predict that the addition proceeds in two steps with the formation of a zwitterionic intermediate.