The partition coefficient for technical grade naphthenic acid in water/n-decane at 295 K has been determined (K(wo) = 2.1 center dot 10(-4)) using a simple experimental technique with large extraction volumes (0.09 m(3) of water). Furthermore, nonequilibrium values at different pH values are presented. Analysis of the acid content in the oil phase was conducted by FT-IR and colormetric titration and found to be equivalent.