A methodology was developed, which helped to investigate the 'light-off' characteristics of a real diesel oxidation catalyst (DOC) while it was connected to a diesel engine. As a demonstration of the technique, trials were performed on a Ford 2.0 L engine, with a Pt on gamma-alumina DOC (o.d. = 106 mm; length = 114 mm). By operating the engine at a constant speed (e.g. 2000 rpm), it was shown that when the engine torque was varied (e.g. from 5 to 47 N m), the gas inlet temperature to the DOC could also be varied (e.g. from 146 to 285 degrees C), while the gas composition and the mass flow of gas remained relatively constant this enabled more scientific studies of catalyst 'light-off' to be performed on a real exhaust system. Experiments were also performed with higher background levels of a reactant (e.g. CO concentrations = 3000, 4000 and 5000 ppm), and hysteresis between 'light-off' and 'light-down' was observed. It was also shown how valuable additional information may be obtained from experiments using a thin-slice DOC (o.d. = 106 mm; length = 5 mm), and the CO inhibition effect was demonstrated on a real engine exhaust. (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.