The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2+center dot) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2+ (m/z 45), loss of a methyl radical to form NHNH2+ (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2+center dot. Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)(2)NNCH3+ (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2+center dot. The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (Delta H-f(0)) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E-0) obtained from the fitting procedure: Delta H-f(0)[(CH3)(2)NNCH3+] = 833 +/- 5 kJ mol(-1), Delta H-f(0) [C3H8N2+center dot] = 1064 +/- 5 kJ mol(-1), Delta H-f(0)[CH2NHNH2+] = 862 +/- 5 kJ mol(-1), Delta H-f(0)[NHNH2+] = 959 +/- 5 kJ mol(-1), and Delta H-f(0)[N2H2+center dot] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to hv approximate to 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.