The light stabilities of two commercial colorants for Cheddar cheese, an annatto extract and a β-carotene preparation, have been compared in a model system and tested in industrial scale production. The apparent quantum yields for photodegradation by monochromatic light were determined at 25 °C for solutions of the colorants in air-saturated 10 mm-citric acid–disodium hydrogen phosphate buffer containing 100 mg sodium caseinate/l at pH 5·2 and 5·4. Solutions of commercial β-carotene were more photostable than solutions of annatto, and although the quantum yields for photodegradation for both solutions depended significantly on both pH and irradiation wavelength, exposure to u.v. light (313 and 366 nm) caused more photobleaching than exposure to visible light (436 nm). The differences in colour stability observed in the model system were verified by tristimulus colorimetry during 9 weeks illuminated storage at 5 °C of Cheddar cheese produced from milk to which had been added (per 1000 kg) 67 g β-carotene extract or 133 ml annatto.