The Medipix device is a silicon pixelated detector designed by CERN for the detection of different types of radiation. The detector response features can be exploited to identify different types of particles: heavy charged particles, mips, electrons or photons. However, while heavily ionizing charged particles are clearly recognizable, electrons and photons can hardly be distinguished from each other and thus, mistagging of radiation quanta has to be considered. Experiments were performed to quantify the mistag rate in different experimental situations. A GEANT4 simulation of the detector and the experimental setup was used to further understand the results. The detection efficiency for photons was studied and the advantages of using a Timepix device highlighted. (C) 2010 Elsevier B.V. All rights reserved.