ABSTRACT The mercury vapor monitor, MDC-1-01, described in this paper is the first prototype in the MDC series. It was designed and developed in our laboratory to measure airborne mercury vapor on a “continuous” basis in the range of 400 ng to 20 mg mercury per cubic meter of air. In comparison with other instruments, commercially available, it uses much improved principles as regards electronics, optics, and chemistry [l]. The double beam assembly, in combination with a selective filter, permits the measurement of mercury vapor in the presence of the most commonly encountered organic vapors and dusts. Practical applications are discussed.