Intramicellar viscosities of four representative surfactants, SDS, C12E5, C12E6, and DAC, were determined by making use of the intramolecular excimer formation in 1,3-di(1-pyrenyl)propane (DPP) as a fluorescence probe. High-pressure technique was used to change solvent viscosity. To determine the pressure dependence of the rate constant of the excimer formation, fluorescence quantum yield ratio (φIE/φLE) and excimer lifetime (τE) were measured. The intramicellar viscosity of C12E5 and C12E6 showed a quite larger pressure dependence as compared with that of SDS. © 1990 American Chemical Society.