Measurements of fluorescence spectra of solutions at room temperature have been used to study the nature of intermolecular interaction in excited molecular complexes that are unstable in the ground state. It was concluded that (a) when the two molecules are unsubstituted aromatic hydrocarbons or their alkyl derivatives (mixed excimers), the type of inter action seems to be the same as in excimers, i.e., the complex is formed only when resonance interaction is possible and it has zero dipole moment; (b) when one of the hydrocarbons has a donor or acceptor substituent group (exciplexes), the main interaction has a charge-transfer character to which a dipole-dipole interaction may contribute if there is an overlap between the emission spectrum of the excited molecule and the absorption spectrum of the other.