The Cambustion DMS500, a Novel Aerosol Sizing Instrument with Fast Time Resolution, Was First Employed to Sample Jet Engine Particulate Matter Emissions during Project APEX. This Paper Compares the Performance of the DMS500 to that of Traditional Aerosol Instruments for Sampling Jet Engine Exhaust Aerosol under Field Conditions during This Campaign. the Observed Geometric Mean Diameter with Respect to the Particle Number (Dg) Ranged from 15 to 45 Nm, and with Respect to the Mass (Third Moment) Distribution (DgM) from 21 to 112 Nm, the Geometric Standard Deviation (Σg) Ranged from 1.22 to 1.90 and the Total Number Concentration (N) Ranged from 6 X 103 to 3.3 X 105/cm3 (After Dilution). on Average, the Dg, DgM, Σg, and N of the DMS500 Size Distributions Differed by -9, -7, +1, and +30% from the Reference Values of the Traditional Instruments. Compared with the Reference Values, Both Dg and Σg of the DMS500 Showed a Small But Statistically Significant Decrease with Increasing Particle Size. Effects Due to Particle Shape Appeared to Be the Most Likely Explanation for the Observed Size-Related Trends. the 30% Disagreement in Concentration Measurements is Reasonable When the Sensitivity of the 3022 Condensation Particle Counter to Pressure Fluctuations Encountered during Measurements at the Engine Exhaust Nozzle is Taken into Account.