A mathematical model to represent the startup operation of a continuous parallel plate electrochemical reactor is proposed. The model takes into account the diffusion layer adjacent to the working electrode and the convective region in the solution bulk. Experimental results of copper electrowinning from dilute solutions are compared with theoretical predictions. At short times the results are in close agreement with the transient equation derived from the diffusion layer and at larger times with that from the convective region. The mass transfer performance of the reactor is also analysed.