Recently, an approach involving electrochemical preferential oxidation (ECPrOx) of CO was suggested as having the potential to replace the PrOx concept for deep CO removal front reformate gas in proton exchange membrane (PEM) fuel cells. The first part of this paper deals with the characterization of such an ECPrOx unit from a reaction engineering point of view. Based on a spatially lumped, isothermal model, the qualitative selectivity-conversion behavior is discussed for varying feed flow rates and CO inlet mole fractions. A simple two-phase mechanism is suggested that explains the findings. The second part of the contribution considers qualitative questions on cascading of two ECPrOx reactors. The crucial importance of the configuration of their electrical connection is demonstrated and explained. While two cells connected electrically in parallel exhibit almost the same selectivity-conversion behavior in comparison with a single cell, an electrical series connection enables a considerable increase in the selectivity at the same CO conversion. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3196244] All rights reserved.