This paper reports a mass transfer model of a reactant flowing in a large aspect ratio microfluidic chip made of a channel with electrodes on the side walls. A semianalytical solution to the two-dimensional Fickian diffusion of a reactant in a microchannel, including the electrochemical reaction at the electrode interface and the velocity profile obtained from the Navier-Stokes equations in a fully developed laminar regime, is found. The solution is written in the Laplace domain in terms of transfer functions. The proposed solution is an extension of the Lévêque approximation describing the reactant diffusion from the electrode to the middle of the microfluidic channel. The main applications of this work are the use of the obtained transfer functions for the measurement of the Faradic current density or the chemical concentration at the electrode interface. The study can also be extended to the heat transfer in microfluidic electrochemical chips (temperature or heat flux measurements at the electrode interface).