An exact treatment of the interaction-induced correlations between optical transitions in different molecules is presented. In a pure fluid (or in its buffer solution), the correlation terms are found to be additive with the conventional relaxation matrix elements. Same projection approach is used to study similar effects for molecules belonging to different species a and b. An exact solution derived for binary mixtures is used to analyze the cross (ab)-terms in the case of two-line mixing. At the binary collision regime, new spectral features of higher orders in density may appear due to the intermolecular optical transition coherence.