Electrodes used in electroanalysis, which are based on carbonaceous nanomaterials such as carbon nanotubes or graphene, often exhibit large degrees of porosity. By systematically varying the morphology of functionalized graphene electrodes from nearly flat to highly porous, we demonstrate experimentally that minute amounts of electrode porosity have surprisingly significant effects on the apparent reaction kinetics as determined by cyclic voltammetry, both in the reversible and the irreversible regime. We quantify electrode porosity using a coulometric approach and, with the help of numerical simulations, determine the correlation between electrode pore volume and apparent electrode kinetics. We show that in the reversible and quasi-reversible regime, the voltamperometric response constitutes a superposition of thin film diffusion-related effects within the porous electrode and of the standard flat electrode response. For irreversible kinetics, however, we show that diffusive coupling between the electrode and the electrolyte can, under suitably chosen conditions, result in effective electrocatalytic behavior. Confirming past theoretical work by Compton and others, our experiments demonstrate that for a comparison of electroanalytical data obtained with different electrode materials it is not sufficient to only consider differences in the materials' chemical structure but equally important to take into account differences in electrode morphology.