The vibronic nature of the lowest energy electronic absorption and of the lowest energy photoelectron spectrum of NH3 are investigated theoretically. Two-dimensional Franck-Condon factor calculations are performed and theoretical spectra are constructed including the effect of vibronic linewidth. The comparison of the experimental with theoretical spectra, computed under various assumptions, leads to several conclusions. The conspicuous continuum underlying the Ã ← X̃ transition is seen to arise from the overlapping tails of vibronic line shapes. There is no need to attribute the continuum to a second electronic transition. The ν1, N-H stretching mode, is excited in both the electronic absorption and photoelectron spectrum. The Ã state of NH3 may be able to support two quanta of ν1. The planar NH3+ ion has a N-H bond longer than in the ground state NH3 by about 0.06 A. © 1981 American Institute of Physics.