Appearance energies for molecular and some fragment ions from 3-methyl-3-acetyl-, 1,2-dimethyl-3-acetyl-, 1,2-dimethyl-3hydroxycarbonyl-and 1,3-dimethyl-2-(1-methyl)vinylcyclopropenes and model compounds 2,3-dimethylfuran and 1-methylindene were measured by photoionization mass spectrometry. It was shown that, in all cases, the structure of the [M-Me] + ion does not correspond to the expected structures of substituted cyclopropenium ions. Possible schemes for formation of [M-Me] + ions are given. Isodesmic reactions were systematically applied for calculation of the heats of formation for 35 possible C 5 H 5 O + isomers from 3methyl-3-acetyl-cyclopropene. In addition, the heats of formation for more than 30 other unsaturated ions and free radicals were estimated with the help of series of isodesmic reactions. A new value of 160 kJ mol-1 , as compared with the currently used value of 215.5 kJ mol-1 , for the cyclopropylmethyl free radical is suggested. The publication represents the further development of the methodology for the heat of formation estimation procedure for even-electron ions and free radicals.