We study the fluorescent characteristics of eosin molecules in solutions of bovine serum albumin at various pHs. It is established that the quenching of eosin fluorescence with an increase in its concentration is due to two competing processes: the binding of BSA molecules to eosin molecules and the formation of associates of dye molecules. The degree of association, the structure and thermodynamic parameters of dimers in buffer and BSA solutions at different pH values are determined. Variations in the pH of eosin buffer solutions lead to the change of all these characteristics. It is shown that the formation of eosin associates in BSA solutions leads to a decrease in the dynamic quenching constant of eosin fluorescence, which is proportional to the pH of the solution.