Variable-temperature (-55 to -100 degreesC) studies of the infrared spectra (3500-400 cm(-1)) of methylhydrazine, CH3NHNH2, dissolved in liquid xenon have been recorded. From these data the enthalpy difference has been determined to be 323 +/- 30 cm(-1) (3.86 +/- 0.36 kJ/mol) with the inner conformer (methyl group staggered and between the two hydrogens of the NH2 group) the more stable rotamer. A complete vibrational assignment is presented for the inner conformer, and several of the fundamentals of the outer conformer have been assigned. These assignments are consistent with the predicted wavenumbers obtained from ab initio MP2/6-31G(d) calculations utilizing three scaling factors. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations. Hybrid density functional theory (DFT) calculations to obtain the structural parameters and conformational stability by the B3LYP method were also carried out. These quantities are compared to the corresponding experimental quantities when appropriate. Additionally confomational stabilities and structural parameters have also been predicted from MP2 level calculations with full electron correlation with 6-311+G(d,p) and 6-311+G(2d,2p) basis sets. The r(0) structural parameters have been obtained from a combination of the previously reported microwave rotational constants and ab initio MP2/6-3114G(d,p) predicted parameters. The results are compared to some corresponding quantities for some similar molecules.