A reversible electron transfer is studied at a microdisk electrode which is partially blocked by an inert particle at the disk center. The inert particle is either a cylinder of a sphere. The size of a cylinder located at the center of the microdisk electrode is determined with the help of a simple electrochemical procedure. Cyclic voltammograms for the reduction of a reversible redox system are recorded as a function of scan rate and compared to simulations. Excellent agreement between theory and experiment was found. Modelling diffusion to a microdisk electrode modified with a sphere or cylinder at its center by numerical simulations gives physical support for the transition in diffusion-layer shape and size, therefore allowing to size not only particles of known shape but also, in principle, to determine their shape and size simultaneously. (C) 2007 Elsevier B.V. All rights reserved.