Carbonyl compounds play an important role in atmospheric chemistry, and have adverse effects on human health because they are precursors of ozone and peroxyacyl nitrates. This study investigated the emissions of carbonyl compounds from a heavy-duty diesel engine (HDDE) at one low load steady-state condition (24.5% of the max load at 40 km/h). The experimental results indicate that the emissions of total carbonyl concentrations decreased by 4.7% to 32.3% when 10 to 70 L/min H2/O2 mixture was added to neat diesel fuel. The emissions of nine individual carbonyl species also decreased. Among these, formaldehyde was the major species, accounting for 80.4%–81.5% of the total concentrations in the exhausts of all tested fuels, and its emissions were reduced from 5.1% to 31.7%. Meanwhile, the emission factors of total carbonyl compounds, in mg/L, decreased from 3.6% to 13.0%, and those of total carbonyl compounds, in mg/kWh, decreased from 4.6% to 32.3%. These results indicate that the addition of a H2/O2 mixture to neat diesel fuel can reduce emissions form diesel engines to the atmosphere.