Abstract—The quantum yield of the photodynamic inactivation of lysozyme increases in the sequence acridine orange, methylene blue, proflavine and acriflavine (1:5:6:12). At least up to protein concentrations of 0.1 mM, singlet oxygen is exclusively responsible for the inactivation of the enzyme. For methylene blue, acriflavine and proflavine the quantum yields decrease considerably with increasing dye concentrations. From measurements in H2O and D2O buffer solutions it was concluded that in the case of methylene blue the effect is mainly caused by the quenching of singlet oxygen [rate constant (3–4) × 108M−1s−1]. For the acridine sensitizers both singlet oxygen and dye triplet quenching processes have to be taken into consideration. It has been found that all sensitizers act as competitive inhibitors of the enzymatic reaction of lysozyme. However, the dye‐protein interaction near the active center cannot be responsible for the observed dye self‐quen