Theoretical expressions are given for the ac impedance of channel microband electrodes operating under laminar flow conditions. The effect of electrode size and solution flow rate is examined and the relative roles of axial diffusion and convective mass transport are identified so that the electrode impedance is quantified over the full experimental operating range of any practical electrode. Experiments employing the oxidation of ferrocene in acetonitrile solution at platinum microband channel electrodes are reported and shown to be in good agreement with theory.