An in situ particle flux monitor (High Yield Technology, Inc., model PM-100) was evaluated using monodisperse polystyrene latex and oleic acid aerosols. A conventional laser particle counter with a lower size limit of 0.3 μm was used to obtain efficiency data. The counting efficiency of the monitor was determined experimentally as a function of particle size in the diameter range of 0.5–6.0 μm. Other operating characteristics that were studied include the effect on counting efficiency of particle velocity, spatial uniformity of the viewing plane, aerosol concentration, and discriminator setting. In addition, the application of the monitor in a vacuum environment was demonstrated. Results showed that the monitor counting efficiency decreased with particle size (from 4.7% at 6 μm to 0.1% at 0.5 μm). Also, the counting efficiency was found to be dependent on sampling velocity but was unaffected by particle concentration. Further tests showed that the monitor had limited sizing ability. Mie scattering analysis of the sensor geometry suggested that the sensor performance could be improved by modifying the sensor sampling geometry and laser beam configuration. Although the counting efficiency of the monitor is lower than that of a conventional laser particle counter, the small size of the sensor makes it a useful process-monitoring tool (see Borden et al., 1987).