p-Nitrophenol (PNP) was extracted from 1-octanol into an aqueous buffered solution using membrane-supported extraction in hollow fiber liquid contactors (HFLCs) containing hydrophobic, microporous polypropylene fibers. PNP is a weak acid and in aqueous solution may dissociate to form nitrophenolate ion (PNP-). that has negligible solubility in 1-octanol. The ratio of the two species in aqueous solution is governed by pH; therefore, the overall mass transfer coefficient based on solvent phase concentrations was determined as a function of pH. The extraction of PNP is a four-step process. consisting of diffusion across the solvent boundary layer, diffusion through the solvent-filled membrane, reaction (dissociation) at the solvent/aqueous interface, and diffusion across the aqueous boundary layer. The reaction step is assumed much faster than the others; a model of the mass transfer capabilities of the system is presented based on this assumption. The overall mass transfer coefficients were determined experimentally by recirculating the solvent phase through the shell space of a HFLC and the aqueous phase through the fibers. The model closely predicts the experimentally measured trends of the overall mass transfer coefficient.