Composition distributions of mixed micelles are analyzed using basic principles of classical statistical mechanics with special emphasis on potential application to fluorescence quenching experiments in such systems. The problem is formulated as a lattice model with nearest-neighbor interactions between surfactant hydrophilic moieties. Firstly, the mean-field approximation is discussed in some detail with focus on its general character. Deeper analysis, involving direct evaluation of the lattice partition function, shows how substantial the deviations from the mean-field predictions can be. The situation is dramatically improved when the truncated cumulant expansion is introduced. Two special cases are distinguished, where the so-called interchange energy takes positive or negative values. In the former case, a binary mixture can separate into two phases of different composition, while in the latter micelles can undergo structural ordering. Finally, mixtures with excess of one component are considered and a useful approximation for the grand partition function is derived.