A new concept in reaction-based chemical analysis is introduced and theoretically described. By utilization of the variability in electrophoretic mobilities among charged species, spatially distinct zones of chemical reagents can be electrophoretically merged under the influence of an applied electric field. Electrophoretically mediated microanalysis (EMMA) exploits this phenomenon as a basis for chemical analysis utilizing capillary electrophoretic systems. EMMA is described in terms of the four stages required for reaction-based analysis: (1) analyte and analytical reagent metering; (2) initiation of reaction; (3) control of reaction conditions and product formation; (4) detection of species whose production or depletion is indicative of the concentration or quantity of the analyte of interest. The method is illustrated by the enzymatic oxidation of ethanol to acetaldehyde by alcohol dehydrogenase with the concurrent reduction of NAD+ to NADH monitored at 340 nm. Experimental results for both substrate and enzyme determinations are shown to agree with the presented theory.