Accurate and precise ionization energies of methylamines (CH3NH2 and CH3ND2) are determined to be 9.0422+/-0.0012 and 9.0532+/-0.0012 eV, respectively, by (1+1') two-photon mass-analyzed threshold ionization (MATI) spectroscopy. From selective ionizations from specified intermediate quantum states, fundamental frequencies of amino-wagging and CH3-rocking modes of CH3NH2+ (CH3ND2+) in D-0 states are determined to be 738 (573) and 1013 (1024) cm(-1), respectively. The frequency of the amino wag is largely blueshifted from that of the neutral S-1 state, while the CH3-rocking frequency is little shifted from that of S-1. Internal rotational constants associated with the nearly free internal rotation of the top (amino group) with respect to the frame (methyl group) about the C-N axis are accurately determined, from which the geometries of methylamine ions are revealed. Barrier heights for torsional motion in CH3ND2+ are determined to be 25+/-5 and 34+/-5 cm(-1) at the origin and first ND2-wagging bands, respectively. Mode-resolved spectroscopy using the MATI signal for resolving overlapped spectral features in the intermediate state is employed for clarifying the vibrational assignment of the intermediate state. An ab initio calculation at the QCISD level is carried out, giving good agreement with the experiment. (C) 2003 American Institute of Physics.