We report the mass analyzed threshold ionization (MATI) spectra of p-methoxyaniline recorded via the 0(0) vibrationless and several vibrational levels in the S, state. The adiabatic ionization energies (IE) of this molecule is determined to be 57445 +/- 5 cm(-1), which is red shifted from that of aniline by about 4826 cm(-1). This large energy shift is attributed by the electron-donating nature of the OCH3 group, supported by the ab initio calculations. In addition, the presence of the methoxy substituent also influences the vibrational frequencies depending upon the pattern. (C) 2003 Elsevier Science B.V. All rights reserved.