Analysis of particulate matter (PM) emissions from gas turbine engines, using the conventional smoke number (SN) technique, provides a measure of plume visibility. In this study, PM emissions were sampled from the exhaust of a small gas turbine engine, burning Jet A-1, and Biodiesel. SN results indicated that biodiesel significantly reduced visible emissions. Analysis of PM number and mass concentrations using a differential mobility spectrometer found that although nonvolatile PM was significantly reduced, biodiesel combustion produced a high fraction of volatile PM. Concurrent aerosol mass spectrometer measurements established that the condensable material was organic in composition. The condensation of volatile organics, not captured by the SN technique, significantly increased the total PM emissions. Application of the Society of Automotive Engineers Aerospace Recommended Practice 1179d for gas turbine engines is limited to visible plume characterization and thus is inadequate when combustion produces a large fraction of volatile or non-light absorbing PM emissions.