The structure and properties of CH3NN, the methyldiazenyl radical, have been studied using quadratic configuration interaction (QCI) ab initio calculations. Fully optimized equilibrium and transition state geometries were located at the QCISD/6-311G** level and the energies at these points were evaluated using QCISD(T)6-311G*. The intrinsic reaction coordinate (IRC) for dissociation of the methyldiazenyl radical to form N-2 plus CH3 has been traced at the QCISD/6-31G* level. These IRC results show that extension of the C-N bond is accompanied by shortening of the N-N bond, contraction of the N-N-C angle, and planarization of the methyl group. Dissociation is found to be exoergic and activated, with a small activation barrier whose height is estimated to be 630 cm(-1) in a vibrationally adiabatic process. The standard heat of formation of methyldiazenyl radical is calculated to be 262 kJ/mol. Tunneling calculations based on the computed IRC profile predict the methyldiazenyl radical to be bound, but short lived, with a maximum lifetime between 10(-11) and 10(-9) s.