The substitution of CNG for diesel fuel in heavy-duty vehicles has potential greenhouse gas benefits. To realise this potential the reductions in CO₂ emissions must not be offset by increased emissions of gases with higher global warming potential (e.g.. N₂O and CH₄). The present work was commissioned by the Australian Greenhouse Office as a pilot study to determine N₂O emissions from two dual-fuel trucks operated back-to-back on diesel and then in CNG/diesel mode. Measurements were carried out for four steady state loads, 10%, 50%, 75% and 100% of maximum, but valid data using CNG was confined to one vehicle. Analyses were carried out continuously using an analyser specific to N₂O and off-line by FTIR spectroscopy of filtered bag samples. The FTIR spectra were also processed to obtain CO and CH₄ concentrations. Concentrations measured for N₂O were always very low with ‘net production’ (i.e. above the ambient concentration of 0.31°ppm) averaging 0.49°ppm by the analyser and 0.41°ppm by FTIR. However the accuracy of individual measurements was insufficient to establish any dependence on load or on the fuel in use. For the concentrations observed the contribution of exhaust N₂O is negligibly small compared with that of CO₂ in terms of global warming. The FTIR spectra yielded CO concentrations, and high CH₄ concentrations, with good precision. Exhaust CH₄ concentrations were extremely low during operation on diesel fuel alone with measured values little above the background atmospheric concentration of 1.8°ppm. They were high when CNG was used with concentrations sufficient to offset the global warming benefit of lower CO₂ emissions. CO emissions were also much higher (470 to 900°ppm) when CNG was in use than with diesel fuel alone (25 to 150°ppm).