The influence of the medium on the quantum yields of singlet oxygen (O 2 ( 1 Δ g )) production (Φ Δ ) by a series of photosensitizing non-steroidal anti-inflammatory drugs (NSAID) derived from 2-arylpropionic acid (APA) has been investigated. Four-component oil-in-water and water-in-oil microemulsions, based on anionic and cationic surfactants, have been employed as the simplest models to mimic more complex biological environments. Φ Δ values have been determined by monitoring the singlet oxygen ( 1 O 2 ) luminescence at 1270 nm upon continuous excitation of the drugs under air-equilibrated conditions. Results indicate that Φ Δ values are highly affected by the medium, being higher in microheterogeneous systems than in (homogeneous) solution. Some of the anti-inflammatory derivatives are very efficient 1 O 2 sensitizers: e.g., values of apparent Φ Δ as high as 0.86 (±0.04) and 0.70 (± 0.03) have been found for triaprofenic acid and suprofen, respectively. The location of the drugs in the interfacial region of the microemulsions combined with their high Φ Δ values suggest that type II reactions may play a significant role in the overall photodynamic process in more complex organized media, such as biological membranes.