The influence of the medium heterogeneity upon the bimolecular rate constants for the physical quenching, k_q, and chemical quenching, k_r, of singlet molecular oxygen O₂(¹Δ_g) by seven natural and three synthetic carotenoids (CAR) with different substituent patterns was studied in a reverse micelle system of sodium bis(2-ethylhexyl)sulfosuccinate, hexane and water. Because O₂(¹Δ_g) was generated inside the water pools of the reverse micelles by photosensitization of the water-soluble dye rose bengal and the CAR are mainly located in the external hexane pseudophase, the quenching process was interpreted using a pseudophase model for the partition of O₂(¹Δ_g) between the water pools and the organic pseudophases. The k_q values were mainly dependent on the extent of the double-bond conjugation of the CAR, as demonstrated by a good empirical relationship between log(k_q) and the energy E(S) of the longest wavelength transition π→π* of the CAR. In contrast, the k_r values were almost independent of the extent of the double-bond–conjugated system and about four orders of magnitude lower than k_q. However, in all cases, CAR photobleaching was observed with the formation of various oxidation products, depending on the photosensitization time. Chromatographic and spectroscopic product analysis for the reaction products of β-carotene with O₂(¹Δ_g) indicated the formation of the β-carotene-5,8-endoperoxide as the primary oxidation product.