A mathematical model of a sandwich membrane biosensor operating in the diffusion-limited regime is proposed. The enzyme (oxidase) is immobilized between two diffusion membranes with different thicknesses. The electrochemically active product of the enzymatic reaction ( H202) is detected on a catalytic electrode. The exact transient solution of the model is obtained under the assumptions of diffusion-limited enzymatic and electrochemical reactions, a stepwise increase of the substrate concentration (from 0 to Co') and zero concentration of the electrochemically active product (H202) in the bulk solution. The thickness of the enzyme layer in the sandwich is assumed to be negligibly small in comparison with that of the diffusion membranes. The numerical values of the transient concentration distribution C(x, t), of the transient current i(t) and its time derivative di(t)/dt are computed and presented in dimensionless form for different sets of parameters of the model. The influence of the enzyme layer position in the sandwich membrane and of the ratio of the effective diffusion coefficients of the substrate and the electrochemically active product on the transient behaviour of the sensor is discussed. The model gives some hints for the design of amperometric biosensors.