On-board hydrogen production by exhaust-assisted fuel reforming offers a potential route to diesel emission control. In this study, different reformer configurations with the same catalyst composition were tested, as well as three types of diesel engine fuel, under conditions designed to simulate low- and high-load engine operating modes. The reforming efficiency was dependent on the fuel type and followed the general trend of bioethanol > rapeseed methyl ester > low-sulfur diesel fuel. In each case, a characteristic axial temperature profile was established in the catalyst. The profile can be deconvoluted into three phases, corresponding to the consecutive occurrence of combustion, steam reforming, and water-gas shift. The relative contribution of each process not only responds to changes in the inlet conditions but also to the aspect ratio and form of the catalyst bed. The heat produced in the first phase must be effectively transferred to the endothermic and much-slower steam-reforming reaction, whereas in the final phase, heat must be lost to ensure that the thermodynamically limited water-gas shift reaction is favored.