Microchip devices integrating eletrophoretic separations, post-column enzymatic derivatization reactions, and amperometric detection, have been developed. The performance of the new integrated microfabricated chip system is demonstrated for on-chip assay of amino acids based on their electrophoretic separation, post-column reaction with amino-acid oxidase and amperometric detection of the hydrogen peroxide product. Factors influencing the response are examined and optimized, and the analytical performance is characterized. The concept can be extended to different target analytes based on the post-column reactions of other "class" enzymes.