We report the mass analyzed threshold ionization (MATI) spectra of p-ethylaniline recorded via the 0(0) vibrationless and the 6a(1), 11,12 and 121 vibrational levels of S, state. The adiabatic ionization energies (IE) of this molecule is determined to be 59 959 +/- 5 cm(-1) and most of the active vibrations of its cation are related to the in-plane ring vibrations. Comparison among the present results and those of aniline, p-methylaniline, and p-propylaniline shows that the IE decreases as the length of the alkyl chain increases. In addition, the long chain effect on the frequencies of the observed in-plane ring vibrations is insignificant. These experimental findings are supported by ab initio and density functional theory (DFT) calculations. (C) 2002 Elsevier Science B.V. All rights reserved.