The valence shell electronic structures of methylhydrazine (CH3NHNH2), 1,1-dimethylhydrazine ((CH3)(2)NNH2) and tetramethylhydrazine ((CH3)(4)N-2) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+ G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IEa = 8.02 +/- 0.16 eV and IEv = 9.36 +/- 0.02 eV for methylhydrazine, IEa = 7.78 +/- 0.16 eV and IEv = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IEa = 7.26 +/- 0.16 eV and IEv = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IEa values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta H-f) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta H-f for tetramethylhydrazine is shown to be erroneous.