A methodology was developed, which enables transient experiments to be performed on a DOC connected to a live diesel engine, while the temperature of the DOC substrate is monitored at various locations. This was demonstrated on a full-scale Pt/γ-alumina DOC (o.d. = 106 mm; length = 114 mm), connected to a Ford 2.0 L diesel engine. The engine was operated at a constant speed of 2000 rpm, and by varying the engine torque, experiments were performed at fixed gas inlet temperatures to the DOC, e.g. 150, 320 °C, and at each setting a constant background level of exhaust emissions was obtained. Then, a known quantity of pollutant (e.g. CO, propane) was injected into the exhaust, so as to create an approximated pulse input into the DOC (e.g. with peak CO concentration = 3000 ppm). The transient response of the system was monitored. So as to examine more closely local changes in response to a change in inlet conditions, experiments were also performed with a short 5 mm thin-slice of DOC (o.d. = 106 mm). These techniques enable better control to be achieved over such experiments, and provide valuable data to support mathematical modelling. It was interesting to note, that in the examples studied, there was no evidence of any significant competition between CO and THCs for the active sites. The DOC was also characterised, and it was shown that key properties can have different values depending on the location from which the sample was taken - and this would also have important implications on modelling work.