The spectroscopic properties of a series of 1-aminonaphthalenes were examined in the isolated gas phase by means of laser-induced fluorescence combined with supersonic beam techniques. The aminonaphthalenes differ with respect to the size of the amino group, 1AN (NH2), 1MAN (NHCH3), 1DMAN (N(CH3)(2)), or the presence of a second substituent in the 4-position: 4MDMAN (CH3) and 4CIDMAN (Cl). Whereas the fluorescence excitation spectra of 1AN and 1MAN show an intense 0-0 transition and a well-resolved vibrational pattern, that of the N,N-dimethylamino derivative 1DMAN displays a particularly complex structure, with a very weak origin and a sudden collapse of intensity at excitation energies above 1000 cm(-1). A long low-frequency vibrational progression, which has been assigned to the torsional mode of the (N(CH3)(2)) around the C-N bond, clearly appears in the spectra of the 4-substituted derivatives. DFT calculations of the ground-state structure of 1DMAN and 4CIDMAN show that the dimethylamino group is twisted by 49.7degrees relative to the aromatic plane. An analysis of the Franck-Condon distribution along the torsional progression for 4CIDMAN indicates a decrease of the dimethylamino twist angle in S-1 by about 20degrees. The energy threshold for the drop in fluorescence intensity, depending on the nature of the substituent in the 4-position (1200 cm(-1) for 4MDMAN, 700 cm(-1) for 4CIDMAN), indicates the presence of an efficient internal conversion process, similar to what has previously been observed in solution. Complexation of 1DMAN with acetonitrile reduces the internal conversion efficiency, as can be deduced from the enhanced fluorescence intensity as well as from the lengthening of the fluorescence decay time. The collapse of the FES spectrum of the 1DMAN/ CH3CN complex (1:1) occurs at a significantly higher excess energy (1500 cm(-1)) than in the case of bare 1DMAN.