The effects of methyl substitution on the geometries and bonding energies of a systematic series of three-electron-bonded radical cations of the type [HnX.-.XHn](+), covering all possible symmetrical three-electron bonds that may take place between atoms of the second and third rows of the periodic table, have been investigated at the level of Moller-Plesset perturbation theory. Methyl substitution leads to significant weakening and lengthening of the X.-.X bond when X is a second-row atom. The effects increase with the number of substitutions and are more and more important in the series X = N, 0, F. By contrast, methyl substitution leaves the bonding energies between third-row atoms practically unchanged but leads to a surprising bond shortening in the S..S and P..P cases. These seemingly contradictory effects are rationalized through a qualitative analysis based on an elementary molecular orbital description of three-electron bonding.