The fluorescence excitation spectra and single vibronic level fluorescence spectra of jet-cooled phenanthridine and 7,8-benzoquinoline are reported. These spectra are entirely of molecular origin, as revealed by their dependence on the conditions of supersonic expansion (partial pressure of substances, pressure of He carrier gas). Such conclusion is in general agreement with symmetry-based arguments - contrary to the case of acridine (symmetric monoazaanthracene), the capability of these monoazaphenanthrenes to the formation of homodimers (under jet-cooling connditions) is virtually nil. A preliminary analysis of vibronic features of the first excited singlet state of both monoazaphenanthrenes, based on the comparison with the vibrational assignments for the jet-cooled phenanthrene, and with ground-state IR spectra of these molecules recorded in low-temperature argon matrices are showing that vibrational ground state frequencies are changing upon excitation. This is also confirmed by the vibrational structure of the single vibronic level fluorescence spectra. (C) 2004 Elsevier B.V. All rights reserved.