Intermolecular interaction and stable structures of the p-aminophenol-M (M=CO, N-2) 1:1 complexes have been studied by measuring the S-1 <-- S-0 (pipi*) fluorescence excitation spectra and the IR dip spectra in the OH and NH stretch region combined with ab initio calculations. The S-1-S-0 electronic origin of the CO complex is 141 cm(-1) red shifted from the origin of the monomer. The red shift for the CO complex is smaller than 153 cm(-1) for the N-2 complex, although the molecular polarizability of carbon monoxide is larger than that of molecular nitrogen. The OH stretching frequency of the CO complex is 26 cm(-1) red shifted from that of the monomer in, the IR dip spectrum, but the N-2 complex shows no shift. On the basis of these findings we conclude that carbon monoxide is bonded to the OH group via a hydrogen bond, whereas nitrogen is van der Waals-bonded to the pi cloud of the aromatic ring.