Cytochrome P450 2C9 (CYP2C9) is one of the most important isoforms in human liver involved in the metabolism of a large number of therapeutic agents. The aim of this paper is to demonstrate the applicability of CE for the determination of the enzymatic activity of CYP2C9 with diclofenac as a probe substrate. MEKC with SDS as a pseudostationary phase was used for this purpose. Compared to other assays, the MEKC-based method is rapid, can be automated and requires only a small quantity of enzymes and substrate. Moreover the, enzymatic reaction can be monitored with high sensitivity and repeatability even when the reaction mixture is used for the analysis without any pretreatment. The kinetic study on the given enzymatic reaction was also performed since the basic characterization of drug biotransformation generally begins with the enzyme kinetic analysis of metabolite formation. As a result, the Michaelis constant and maximum reaction velocity were evaluated the, values 3.44 +/- 0.45 gM and 19.78 +/- 0.76 nmol . min(-1)nmol(-1), respectively, were in agreement with the literature data. On the other hand, a slight deviation from typical Michaelis-Menten kinetics with a weak positive cooperativity was found at diclofenac concentrations below 2 gM. The same atypical kinetic behavior of CYP2C9 was also observed by other authors.