The dissociation constant values ((s)(s)pK(a)) of some carbapenem group drugs (ertapenem, meropenem, doripenem) in different percentages of methanol-water binary mixtures (18, 20 and 22%, v/v) were determined from the mobile phase pH dependence of their retention factor. Evaluation of these data was performed using the NLREG program. From calculated pK(a) values, the aqueous pK(a) values of these subtances were calculated by different approaches. Moreover, the correlation established between retention factor and the pH of the water-methanol mobile phase was used to determine the optimum separation conditions. In order to validate the optimized conditions, these drugs were studied in human urine. The chromatographic separation was realized using a Gemini NX C-18 column (250x4.6mm i.d., 5 mu m particles) and UV detector set at 220 and 295nm. Copyright (c) 2013 John Wiley & Sons, Ltd.