A reliable method to assess in vitro metabolic stability of rabeprazole and its modulation by Generally Recognized As Safe (GRAS)-listed pharmaceutical excipients was established in human liver microsomes. The metabolic stability of rabeprazole decreased as a function of incubation time, resulting in the formation of thioether rabeprazole via nonenzymatic degradation and enzymatic metabolism, Buffer type was also a determining factor for the degree of both nonenzymatic degradation and enzymatic and enzymatic metabolism. The net extent of enzymatic drug metabolism, obtained by calculating the difference in drug degradation between a microsome-present reaction System and a microsome-free solution, was about $9.20;{pm}; 0.67$% in phosphate buffer and $2.27{pm}1.76$% in Tris buffer, respectively. Rabeprazole exhibited first-order kinetics in Microsome-free solution but showed non-linear kinetics in the microsome-present reaction system. The maximal velocity, $V_{max}$ in phosphate buffer was 5.07 ${mu}g;mL^{-1};h^{-1}$ and the michaelis-Menten constant, $K_m$, was 10.39 ${mu}g;mL^{-1}$ by computer-fitting to the classical Michaelis-Menten equation for pattern of time-dependent change in the substrate concentration. The intact drug and its thioether form were well resolved and successfully identified by HPLC chromatography and liquid chromatography mass spectroscopy (LC/MS). The metabolic stability of rabeprazole was also modulated by the presence of pharmaceutical excipients. Among the five pharmaceutical excipients tested, poloxamer 188 and Gelucire 44/14 had potentially inhibitory effects on rabeprazole metabolism in human liver microsomes (p < 0.05). A greater understanding of metabolic stability and its modulation by pharmaceutical excipients would be useful for optimizing the bioavailability of rabeprazole at the early formulation stages.