The response characteristics of scattered light aerosol monitors used in environmental control applications were evaluated. The instruments chosen for the study were as follows: a RAM-S (GCA Technology Division, Bedford, MA), a PCAM-TX (ppm, Inc., Knoxville, TN), and an ATI-722 (Air Techniques, Inc., Baltimore, MD). The aerosol test chamber was a hexagonal box equipped with appropriate vacuum and electrical connections. None of the instruments had a significant short term zero drift. The PCAM-TX and RAM-S followed what appeared to be first order response functions. The ATI-722 responded almost immediately when the purge was switched off. RAM-S and ATI-722 began to respond immediately to the change in concentration. Only RAM-S was small enough to be placed inside the chamber. The mass median diameters (MMD) for coal dust-I (CDI) and coal dust-II (CDII) were 5.4 +/- 0.6 and 3.1 +/- 0.3 microns, respectively. All three instruments displayed better linearity for CDI than for CDII. The size sensitivity was determined by passing a test aerosol through a series of impactor stages and measuring the mass response of each instrument relative to the indicated mass response of an aerodynamic particle sizer (APS). The linearity was measured from 0.5 to 10mg/m3. The mass sensitivity of RAM-S was low for particles greater than 9 microns in diameter. The sensitivity increased by about a factor of ten over the range studied. The response of ATI-722 was similar to RAM-S. The PCAM-TX and APS showed similar MMD values. The instrument response for different size distributions was calculated which allowed for determination of the response factor for an optically similar dust when the response factor for one dust was known. The authors state that these instruments are especially useful in applications where time response is important and where exact agreement with gravimetric mass is not required.