Hydrogentrimethylammonium, (CH3)(3)NH., 1, and its deuterium-labeled isotopomers are generated by collisional neutralization with dimethyl disulfide of the corresponding stable cations and characterized by neutralization-reionization mass spectrometry and ab initio calculations. Hypervalent radical 1 dissociates extensively by losses of hydrogen and methyl, which are calculated to be exothermic by 69 and 103 kJ mol(-1), respectively. A small fraction (similar to 0.5%) of 1 is metastable and survives for 3.7 mu s to be detected as stable ions after collisional reionization with oxygen. An unusual inverse deuterium isotope effect on dissociations of 1 is found that stabilizes (CH3)(3)NH. and (CHD2)(CH3)(2)NH. against (CH3)(3)ND.. MP2(FULL)/6-31+G(d) calculations find a very shallow potential energy minimum for the (2)A(1) electronic ground state of 1, which is predicted to dissociate completely by loss of hydrogen. A potential energy barrier is found for the loss of methyl from the (2)A(1) ground state of 1. The existence of metastable 1 and the methyl loss are explained by low-lying bound excited states which are accessed on collisional neutralization.