Poisoning of Pt anode electrocatalysts by carbon monoxide (CO) is deemed to be one of the most significant barriers to be overcome in the development of proton-exchange membrane fuel cell systems (PEMFCs). The use of CO-tolerant electrocatalysts serves as the most hopeful way to solve this problem. It is well established that Pt-based alloy systems of CO-tolerant electrocatalysts can substantially withstand the presence of CO in the fuel stream. Based on literature starting in 2000, a few efforts have still been conducted at developing a more CO-tolerant anode electrocatalyst than the traditional Pt/C or PtRu/C systems. This review introduces and discusses these efforts. Pt-based electrocatalysts, including PtSn/C, PtMo/C (atomic ratio = 5:1), PtRuMo/C (Mo = 10 wt.%), PtRu-HMoO3/C and PtRu/(C nanotubes), appear to be poisoned by CO at the same, or a lower, level than traditional Pt/C or PtRu/C electrocatalysts. Platinum-free electrocatalysts, such as PdAu/C, have proven to be less strongly poisoned by CO than PtRu/C counterparts at temperatures of 60 degrees C. A greater tolerance to CO can be achieved by modifying the structure of the electrocatalyst. This involves the use of a composite or double-layer that is designed to make the CO react with one of the electrocatalyst in advance while the main hydrogen reacts at another layer with a traditional Pt/C electrocatalyst. (c) 2005 Elsevier B.V. All rights reserved.