The development and optimisation of an ultra-fast response chemiluminescence NOx analyser, equipped with a high temperature stainless steel, nitrogen dioxide (NO2) to nitric oxide (NO) converter, for the transient measurement of the NOx (NO + NO2) content of automotive emissions is described. Conventional analysers routinely used to measure NOx in automotive exhaust utilise chemiluminescence detection (CLD). Ultra-fast time-scale analysis using CLD, however, has traditionally been hindered by the slow conversion of NO2 to NO. The converter and technology used in the instrument described herein enables a 10-90% response time of less than 10 ms.Following optimisation in the laboratory, the fast response CLD NOx analyser was evaluated for raw exhaust sampling of port fuel injected (PFI) gasoline and diesel vehicles. Conventional CLD and on-line speciation systems, such as chemical ionisation mass spectrometry (CIMS), were used to assess the effectiveness of the fast response CLD NOx analyser under different exhaust composition regimes.The performance of the ultra-fast response CLD NOx analyser under most operating conditions was found to be excellent. An exception is the lean-burn PFI gasoline engine equipped with a close-coupled three-way catalyst (TWC) and underfloor lean NOx trap (LNT) where, under periodic rich purge regenerations, the ultra-fast CLD NOx analyser exhibits a response to interfering components associated with the NOx converter.