Diffusion limitations due to triiodide transport through nanoporous TiO2 networks are investigated with the help of a simple device structure, a nanoporous TiO2 layer directly sintered onto a platinum electrode and immersed in an I-3(-)/I- electrolyte solution. At voltages below 0.3 V, electronic transport in this device is dominated by the catalytic reaction at the Pt/electrolyte interface whereas above approximately 0.4 V current flow is limited by diffusion of triiodide through the nanoporous TiO2 network. Through the analysis of devices with different TiO2 layer thicknesses and I-2 concentrations we calculate a diffusion constant for I-3(-) of 1.24 (+/-0.02) x 10(-5) cm(2) s(-1) in the bulk of the electrolyte and an effective diffusion constant of 0.44 (+/-0.02) x 10(25) cm(2) s(-1) within the pores of the TiO2 network. (C) 2003 The Electrochemical Society.