Hypervalent dimethylammonium radical, (CH3)(2)NH2. and its deuterium-labeled isotopomers (CH3)(2)ND2., (CH3)(2)NHD., (CD3)(2)NH2. and (CD3)(2)ND2. were generated as transient species by collisional neutralization of their cations in the gas phase and studied by neutralization-reionization mass spectrometry, laser photoexcitation, and ab initio theory. (CH3)(2)ND2., (CH3)(2)NHD., and (CD3)(2)NH2. gave fractions of metastable species of greater than or equal to 3.3 mu s lifetimes, whereas (CH3)(2)NH2. and (CD3)(2)NH2. dissociated completely on the same time scale. Metastable (CD3)(2)NH2. and (CH3)(2)ND2. were photoexcited but not photoionized with the combined 488 and 514.5 nm lines from an Ar-ion laser. Ab initio calculations with effective PMP4(SDTQ)/6-311++G-(3df,2p) identified the ((X) over tilde)(2)A(1) ground state of vertical ionization energy, IEv=3.70 eV. RRKM calculations on the ab initio potential energy surface of the ((X) over tilde)(2)A(1) state predicted predominant N-H and N-D bond dissociations but did not allow for competitive loss of CH3 or CD3. The four lowest excited states of (CH3)(2)NH2., ((A) over tilde)B-2(1), ((B) over tilde)(2)A(1), ((C) over tilde)B-2(2), and ((D) over tilde)(2)A(1), were characterized by CIS/6-311++G(3df,2p) calculations, and their vertical ionization energies were calculated as 2.86, 2.57, 2.48, and 1.82 eV, respectively. The excited states were calculated to be strongly bound with respect to N-H bond dissociations. The N-C bond dissociations were interpreted by potential energy surface crossing of the B and A states and transitions via conical intersection to the dissociative ground state.