This paper presents a modelling study of the ion current formed in a direct-injection diesel engine via chemical kinetics in order to investigate the strong correlation observed experimentally between tailpipe NOx and the ion current. An established n-heptane mechanism augmented with ionic and NOx sub mechanisms is used to model the formation of ions and NOx in three combustion systems, including a phenomenological diesel engine model whose validation is also presented here. Sensitivity analyses are conducted on electron and NOx formation to determine species and reactions common to both. The O radical and, by extension, the mixture temperature are found to be important to both NOx and electron concentrations via mechanisms identified here. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.