Abstract: The interaction between water insoluble drug naproxen with caffeine, nicotinamide and salicylic acid in aqueous solutions were determined by speetrophotometry. Naproxen was found to form 1:1 molecular complex with these substances. The complex formation constants were determined by spectrophotometry at room temperature.The thermodynamic functions associated with complexation of naproxen-drugs were also evaluated. It may be concluded that the complex formation may be attributed to hydrogen bonding since enthalpy changes were shown to be a few KJ·mol^-1.