Western Kentucky University (WKU) recently established a mobile laboratory for monitormg mercury emissions (MMEML). The lab contains facilities to perform both continuous emissions monitoring and the Ontario Hydro Method for mercury analysis. Among the instruments available in the lab are a semicontinuous mercury emissions monitor (SCEM), pretreatment and speciation unit for the SCEM, and an atomic absorption spectrometer with automated sampler. The MMEML was recently utilized at a power plant site that had a 100-MW, wall-fired combustor with low-NO, burners. At this site, a comparison between OHM and SCEM data was possible for testing locations before and after the ESP. OHM and SCEM produced analogous results for the measurement of total mercury, but differ in their measurement of mercury speciation. Testing by OHM also showed that vapor-phase mercury decreases as temperature decreases and as fly ash is removed. Our results suggest that the removal of vapor-phase mercury by fly ash is mostly the removal of oxidized mercury. (C) 2004 Elsevier B.V All rights reserved.