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The addition of oxygenates to diesel fuel reduces particulate emissions, but the mechanisms responsible for the reductions are not well understood. Measurement of particulate matter (PM), unburned hydrocarbons (HC), and carbon monoxide (CO) are routine, but determining the origin of the carbon atoms that make up these undesired emissions is difficult. The sub-attomole (<6×105 atoms) sensitivity of accelerator mass spectrometry (AMS) for measuring carbon-14 (14C) allows tracing the carbon atoms from specific fuel components to soot or gaseous emissions. Radioactive materials are not required because contemporary carbon (e.g., ethanol from grain) has 1000 times more 14C than petroleum-derived fuels. The specificity of the 14C tracer and the sensitivity of AMS were exploited to investigate the relative contribution to diesel engine PM, CO, and CO2 from ethanol and diesel fractions of blended fuels.