The validation of an UV spectrophotometric analytical methodology for quantification of naproxen (NAP) in aqueous media of pH 1,2 and 7,4, and ionic strength 0,15 mol L –1 was developed. The validated methodology is intended to NAP liquid/liquid partitioning studies. UV spectrophotometry was employed because NAP has chromophoric groups (naftyl and carbonyl) thus, allowing good absorption in the UV region. The validated parameters in each pH studied were the following: specificity, linearity, measurement instrument reproducibility, method reproducibility, and intermedium precision. The NAP partitioning at both pH values at 25,0 °C in several liquid/liquid systems was also studied.