Abstract—The chemical modification of rabbit muscle aldolase by coupling with diazotizedp‐amino benzoate results in the preparation of a photosensitive enzyme. The photosensitivity is realized with the presence of arene diazothioether chromophores, due to the substitution on cysteine residues (eight groups per enzyme molecule). By absorption and emission spectroscopy it has been shown that at neutralpH visible light causes E→Z isomerization of the extrinsic chromophore. The reaction is thermally reversed with τ= 2.3 min at 24.3°C. Fading of the chromophore is observed after long irradiation time, especially in alkali solution; atpH 13 some of the diazo groups are exchanged with histidine residues to form azo derivatives of the imidazole anion. Optimum condition for photochromic behaviour of azoaldolase is irradiation with visible light centered at 450 nm, by using neutral solutions of the protein. Within the photochromic cycle slight differences have been observed in the gel electrophoretic behaviour of the azo