Nitrogen monoxide (NO), nitrogen dioxide (NO2) and ozone (O3) have been measured downwind (0–450 m) from two line sources in a suburban environment in a set of initial experiments designed to investigate the role of chemistry in determining the partitioning of NOx into NO and NO2 and the ability of a commercial modelling package (ADMS) to reproduce downwind speciated NOx concentrations. NO concentrations decrease more rapidly than NO2 as NO is removed both by dilution and by chemical reaction with ozone. Whilst NO is the dominant form of NOx at the roadside, NO2 concentrations decay more slowly (due to chemical production and higher background levels) and NO2 typically forms the majority of NOx by 50–100 m from the source. For a majority of modelling studies the input NO and NO2 concentrations were set to match the experimental concentrations at the initial site. The measured decays of NO and NO2 can be accurately reproduced by the ADMS dispersion model only when chemical transformation is taken into account. When NOx is estimated directly from traffic flow measurements, the modelled NO2 tends to underestimate the measured value.