The adiabatic ionization energy of methyl p-aminobenzoate has been determined to be 63549 +/- 5 cm(-1), which is greater than that of aniline by 1278 cm-1. This blue shift results from the COOCH3 substitution at the para position and is consistent with that predicted by the GAUSSIAN-3 calculations. Analysis on the cation spectra shows that the active vibrations include in-plane modes 9b, 6b, 12 and 1 with frequencies of 476, 599, 803, and 831 cm(-1), respectively. (c) 2006 Elsevier B.V. All rights reserved.