Secondary deuterium isotope effects (IEs) on basicities of isotopologues of trimethylamine have been accurately measured by an NMR titration method applicable to a mixture. Deuteration definitely increases the basicity, by approximate to 0.021 in the delta pK per D. The IE is attributed to the lowering of the OH stretching frequency and zero-point energy by delocalization of the nitrogen lone pair into the C-H antibonding orbital. Because this depends on the dihedral angle between the lone pair and the C-H, a further consequence is a preference for conformations with H antiperiplanar to the lone pair and D gauche. This leads to a predicted nonadditivity of IEs, which is confirmed experimentally. It is found that the decrease in basicity, per deuterium, increases with the number of deuteriums. The nonadditivity of Es is a violation of the widely assumed Rule of the Geometric Mean.