An elemental carbon based thermo/optical based method for monitoring occupational exposures to particulate diesel emissions was developed. Filters containing collected samples of diesel exhaust particles were placed in the oven of a custom built thermo/optical analyzer where they were volatilized in a pure helium atmosphere using a protocol in which the temperature was stepped up to around 820 degrees-C. The evolved carbon was swept into a second oven where it was catalytically oxidized to carbon-dioxide in a bed of granulated manganese-dioxide that was held at 900 degrees. The carbon-dioxide was then swept into a nickel/fire brick methanator, maintained at 450 degrees, where it was reduced to methane and quantitated as methane by a flame ionization detector containing a special optical filter. Adjusting the transmittance of this filter enabled corrections to be made for pyrolytically generated carbon or 'char' which was formed during the analysis of some materials. Calibration plots constructed using organic carbon standards were linear for filter loadings up to 50 micrograms per square centimeter (microg/cm2) carbon. The detection limit was 0.15microg/cm2 which was equivalent to a carbon concentration of 2microg per cubic meter (microg/m3) for air samples collected for 8 hours at a flow rate of 2 liters per minute. Two or three replicate determinations of particulate samples generated from standard or reference materials such as lignite, sub bituminous coal, and anthracite performed by three different laboratories yielded relative standard deviations of 0.89 to 6.51%. Interference tests performed with carbonates and carbonized materials showed that these did not interfere with the procedure. When the procedure was used to monitor various worksites such as in the trucking industry or 'dieselized' coal mines, elemental carbon particulate exposures ranging up to 500microg/m3 were found. In most cases, however, the exposures were below 100microg/m3. The authors conclude that the thermo/optical technique is selective, practical, and inexpensive. It is well suited for exposure monitoring and evaluating methods for controlling exposures to diesel particulate matter.