The water-gas shift reactor in the fuel processing unit of a fuel cell system has the vital function of reducing the concentration of CO in the reforming reactor's product gas to values of between 1.0 and 1.5 vol% in order to protect the anodic catalyst from becoming irreversibly poisoned. This paper presents Jiilich's recent development in this field, specifically the WGS 6 in the 5 kWe class. The WGS 6 is characterized by a fundamentally new concept for arranging high temperature and low temperature shift stages. Both stages are now coaxially integrated in one joint casing to provide higher values for the power density and specific power, whereas in earlier reactor generations, these stages are arranged in two separate, parallel housings. In addition, this contribution presents results from a long-term experiment for 5000 h on stream with WGS 6 and discusses the temporal trends of the product gas composition and reactor temperatures across this timespan. For this experiment, the inlet gas stream is produced by an autothermal reformer, which is installed upstream of the WGS 6. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.