The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-Right instrument frequently used to measure the size distribution of an aerosol. However, if the Aerosizer＇s counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate. Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer＇s counting efficiency. To calculate counting efficiency, the number of particles between 0.3 and 10 mu m recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor. Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency. Counting efficiencies were less than 1 % for particles smaller than 0.45 mu m, and more than 100% for particles larger than 7 mu m. Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles. Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second. The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems. Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.