ABSTRACT: We have measured the quantum yields (F_D) of singlet oxygen (¹O₂) production by a series of photosensitizing anti-inflammatory agents monitoring the ¹O₂ luminescence at 1270 nm upon excitation of the drugs. The experiments have been carried out in selected four-component anionic microemulsions under air equilibrated conditions. We have assessed the influence of the medium on the efficiency of ¹O₂ production, since microheterogeneous systems may be employed as simplest models to mimic more complicated biological microenvironments. Our result have shown that F_D is highly influenced by the medium and is enhanced in microheterogeneous systems, some anti-inflammatory drugs being very efficient ¹O₂ sensitizers (F_D^app of tiaprofenic acid ~ 0.86 and F_D^app of suprofen ~ 0.70). The location of the drugs in the interfacial region together with their high value of F_D indicate that type II reactions may play a significant role in the overall sensitization process in biological microenvironments.