The 1,1-dimethylhydrazine ion ((CH3)(2)NNH2+center dot) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at RN-C = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: RH-C = 2.298, 2.198, and 2.098 A. The Delta S(45) value, at an internal energy of 2.32 eV and a bond distance of RN-C = 4.265 A, is 65 J K-1 mol(-1). As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at RN-C = 2.565 A, the Delta S decreases to 29 J K-1 mol(-1). For the hydrogen atom loss channel the variation in Delta S is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, Delta S(59) = 26-16 J K-1 mol(-1) over the studied internal energy range. The 0 K enthalpies of formation (Delta H-f(0)) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E-0) obtained from the fitting procedure: Delta H-f(0)[CH3NNH2+] = 906 +/- 6 kJ mol(-1) and Delta H-f(0)[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol(-1). The calculated G3 values are Delta H-f(0)[CH3NNH2+] = 911 kJ mol(-1) and Delta H-f(0)[(CH3)(CH2)NNH2+] = 825 kJ mol(-1). In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.