The effect of common organic solvents on the activities of various human cytochromes P450 has been reported. However, very little is known about their influence on CYP2B6 and CYP2C8 enzymes. The purpose of this study was to investigate the effect of solvents on the kinetics of representative CYP2B6 (bupropion hydroxylase) and CYP2C8 (paclitaxel hydroxylase) reactions in human liver microsomes. Methanol, ethanol, dimethyl sulfoxide (DMSO), and acetonitrile were studied at increasing volumes (v/v). Acetonitrile, DMSO, and ethanol were shown to increase the Kmand decrease the intrinsic clearance (CLint) of CYP2B6-mediated bupropion hydroxylation in a concentration-dependent manner. These solvents did not noticeably alter the Vmaxat concentrations of ≤1% (v/v). Unlike the other solvents studied, the effect of methanol (≤0.5%, v/v) on CYP2B6 kinetics was negligible. Both DMSO and ethanol increased the Kmand decreased the CLintof CYP2C8-mediated paclitaxel hydroxylation in a concentration-dependent manner. Acetonitrile had minimal influence on CYP2C8 enzyme kinetics at concentrations of ≤1% (v/v). Methanol decreased the Kmof paclitaxel at low concentrations followed by an increase at concentrations of ≥2% (v/v). This differential influence on Kmresulted in an increased CLintat low concentrations followed by a decrease at high concentrations. The studied solvents had minimal influence on Vmaxof paclitaxel. Collectively, DMSO and ethanol were not suitable for characterizing CYP2B6- and CYP2C8-mediated reactions because they showed concentration-dependent inhibition. Methanol and acetonitrile at concentrations of ≤0.5% and ≤1% (v/v) appeared to be suitable for the measurement of CYP2B6- and CYP2C8-mediated activities, respectively.