Despite the continuously tightening emissions legislation, urban concentrations of nitrogen oxides (NOx) remain at harmful levels. Road transport is responsible for a large fraction, wherein diesel engines are the principal culprits. Turbocharged diesel engines have long been preferred in heavy duty applications, due to their torque delivery and low fuel consumption. Fleet operators are under pressure to understand and control the emissions of their vehicles, yet the performance of emissions abatement technology in real-world driving is largely unquantiﬁed. The most popular NOx abatement technology for heavy duty diesel vehicles is selective catalytic reduction. In this work, we empirically determine the efﬁciency of a factory-ﬁtted SCR system in realworld driving by instrumenting passenger buses with both a portable emissions measurement system (PEMS) and a custom built telematics unit to record key parameters from the vehicle diagnostics systems. Weﬁndthateveninrelativelyfavourableconditions, while there is some improvement due to the use of SCR, the vehicles operate far from the design emissions targets. The archival value of this paper is in quantiﬁcation of real world emissions versus design levels and the factors responsible for the discrepancy, as well as in examination of technologies to reduce this difference.