The radical cations of ethylene ( 1) and of all its methyl derivatives were investigated by ab initio UHF calculations with the 3–21G basis set. The planar (or almost planar) form was calculated to be the most stable for all the radical cations except the tetramethylethylene radical cation, the optimized structure of which is twisted about the olefinic CC bond by 11°. The 90°-twisted geometries were also optimized; even for the symmetrically substituted ethylene radical cations, the 90°-twisted structure, of a lower symmetry in which the positive charge and the unpaired electron are separated on different olefinic carbon atoms, is energetically favorable because of the conical intersection between the ground doublet (D 0) and the first excited doublet (D 1) states at the symmetrical 90°-twisted form. It was found that the rotational barriers of the symmetrical derivatives are similar to that of 1 .+, while those of the unsymmetrical derivatives are lower than 1 .+. This relationship between the substitution pattern and the rotational barrier found for the methyl-substituted ethylene radical cations is expected also to hold for other olefin radical cations.