Cytochrome P450 (CYP) 2C enzymes contribute to the metabolism of about 30% of all drugs. Known polymorphisms of the respective enzymes and drug-drug interactions have a major impact on the efficacy and safety of some CYP2C substrate drugs. In vivo - in vitro correlations including prediction of the effect of such covariates requires quantitative information on enzyme kinetics. In this article there will be a summary of the values of the Michaelis-Menten constant (K(m)), the maximal velocity (V(max)) and the intrinsic clearance (Cl(int); V(max)/K(m)) for 84 substrates (100 reactions) reported to be mediated by CYP2C9 (variant enzymes CYP2C9.1, CYP2C9.2 and CYP2C9.3), CYP2C8 and/or CYP2C19. Particularly contradictory findings for the same reactions call for some standardization in the assessment of enzyme kinetics.