High levels of ab initio molecular orbital theory have been used to reexamine the structures and thermochemical properties of distonic radical cations CH(2)X(+)H and their conventional isomers (CH(3)X(.+)) (X = F, OH, NH2, Cl, SH, and PH2). It is found that a generally satisfactory description of structures is provided by MP2/6-311+G(2df,p) optimized geometries and of zero-point vibrational energies by appropriately scaled MP2/6-311G(d,p) harmonic vibrational frequencies. Energies are calculated using a modification of the G2 procedure. Heats of formation for all species CH(3)X, CH(3)X(.+), and CH(2)X(+)H, ionization energies of CH(3)X, and relative energies of distonic and conventional ions are reported and compared with experimental values. Although there is generally goad agreement between theory and experiment, there are several instances (particularly CH2F+H, CH2Cl+H, and CH2S+H2) where a reexamination of the experimental data appears to be warranted. Recommended heats of formation (Delta H-f 298), obtained by the G2’ procedure, are 965.8 (CH2F+H), 820.6 (CH2O+H2), 852.1 (CH2N+H3), 1050.2 (CH2Cl+H), 971.4 (CH2S+H2), and 887.8 (CH2P+H3) kJ mol(-1).