Negative-ion photoelectron imaging at 532, 392, 355, and 266 nm is used to assign several low-lying electronic states of neutral nitromethane CH3NO2 at the geometry corresponding to the anion equilibrium. The observed neutral states include (in the order of increasing binding energy) the X (1)A' ground state, two triplet excited states, a (3)A '' and b (3)A '', and the first excited singlet state, A (1)A ''. The state assignments are aided by the analysis of the photoelectron angular distributions resulting from electron detachment from the a' and a '' symmetry molecular orbitals and the results of theoretical calculations. The singlet-triplet (X (1)A'-a (3)A '') splitting in nitromethane is determined as 2.90(+0.02)/(-0.07) eV, while the vibrational structure of the band corresponding to the formation of the a (3)A '' state of CH3NO2 is attributed to the ONO bending and NO2 wagging motions excited in the photodetachment of the anion. (C) 2009 American Institute of Physics. [doi:10.1063/1.3256233]