The most important component in catalytic reduction reactions is Hydrogen formed either during the enriched operation in normally vented engines or during the post-injection phase in common rail engines. At sufficiently high cylinder temperatures, dissociation of fuel molecules into radicals triggers rapid reconnection of protons into H2. This process can yield between 0.6% and 2vol% H2. Catalytic NOx reduction in diesel engines is based on the reaction of hydrogen with NOx. Accurate and time-resolved measurements of hydrogen are needed to develop and improve these types of technologies.