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 litre engine, with a Pt on γ-alumina DOC (o.d. = 106 mm; length = 114 mm). By operating the engine at a constant speed (e.g. 2,000 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 °C), whilst 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 = 3,000, 4,000 and 5,000 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.