This paper presents results from the ongoing optimisation of a microchannel steam reformer for diesel fuel which is developed in the framework of the development of a PEM fuel cell system for vehicular applications. Four downscaled reformers with different catalytic coatings of precious metal were operated in order to identify the most favourable catalyst formulation. Diesel surrogate was processed at varying temperatures and steam to carbon ratios (S/C). The reformer performance was investigated considering hydrogen yield, reformate composition, fuel conversion, and deactivation from carbon formation. Complete fuel conversion is obtained with several catalysts. One catalyst in particular is less susceptible to carbon formation and shows a high selectivity. To optimise the reformer geometry, a fifth downscaled reformer was implemented with an increased channel height. In operation the reformer shows advantages in performance and pressure losses in comparison to the original geometry. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.