With the I-aminonaphthalenes 1N5 and IDMAN a fast radiationless process occurs in n;hexane, diethyl ether, and acetonitrile, which is shown to be internal conversion (IC). The IC reaction is slower with 1N4 and much less efficient with 1MAN and 1AN. This IC process is thermally activated and slows down with increasing solvent polarity, due to a larger IC activation energy. In the ground state So, the amino twist angle theta relative to the naphthalene plane increases in the order 1MAN, 1AN, 1N4, 1N5, IDMAN, as derived from absorption and fluorescence spectra, II-I NMR spectra, ground-state dipole moments, and ab initio calculations. For the five l-aminonaphthalenes in the equilibrated S-1 state, the twist angle and the radiative rate constant have similar values. The different IC efficiences of these molecules are therefore determined by the structural differences (amino twist angle) between SI and So. A correlation is found between the IC efficiency in these molecules and the twist angle theta. The IC process to So starts from the equilibrated S1 state, which is vibronically coupled with Sg due to a small energy gap Delta E(S-1,S-2). It is therefore concluded-that the extent of vibronic coupling and the magnitude of the twist angle theta are the determining factors in the IC process.