Mathematical modeling is presented for a combined iontophoretic device and biosensor used to analyze glucose concentration in transdermally extracted fluid that is correlated to blood glucose. This device works as follows : an electric current (iontophoresis) is used to extract glucose across the skin into a hydrogel. Within the hydrogel, the extracted glucose undergoes a reaction with the enzyme, glucose oxidase (GOx), to produce gluconic acid and hydrogen peroxide in the presence of oxygen. The hydrogen peroxide then further diffuses to and reacts on a platinum electrode to produce two electrons, water, and oxygen. The measured electrical current is proportional to the flux of glucose entering the hydrogel. Modeling for this device involves the diffusion of (alpha- and beta-) glucose and hydrogen peroxide in the hydrogel, as well as the reaction of beta-glucose with GOx, the mutarotation of the alpha-, beta-anomeric pair, and the oxidation of hydrogen peroxide on a platinum catalyst/electrode. The reaction-diffusion problem exhibits both axial and radial diffusion as well as discontinuous boundary conditions, requiring the use of finite element methods to perform the simulations.