The emissions of oxides of nitrogen (NOx) from biodiesel blended fuels reported in the literature vary from an NOx increase to an NOx decrease relative to the neat petroleum diesel fuel (PDF). To explain these NOx differences, three PDFs with varying fuel properties were admixed with a neat soy-derived biodiesel at 10 per cent and 20 per cent volume ratios and evaluated using a heavy-duty diesel engine exercised over transient and steady-state cycles. The PDFs with ‘low’ and ‘medium’ cetane numbers led to a change in combustion phasing when blended with the neat biodiesel, resulting in reduced NOx emissions at low engine power. The B100 blended with the ‘high’-cetane-number PDF showed minimal change in combustion phasing and resulted in an NOx increase at all engine loads. The derived peak in-cylinder gas temperature variation correlated with the brake-specific NOx emissions indicating that the thermal NOx formation responds to the addition of biodiesel. The biodiesel blends had an NOx—particulate matter trade-off, also suggesting a thermal NOx effect. The increase in NOx emissions of the biodiesel blends also had a strong correlation with the level of saturated hydrocarbons at high engine power.