Differential optical absorption spectrometry (DOAS) has been used by a number of investigators over the past 10 years to measure a wide range of gaseous air pollutants. Recently OPSIS AB, Lund, Sweden, has developed and made commercially available a DOAS instrument that has a number of features which make the unit attractive for field monitoring studies in both remote and urban areas. The DOAS is composed of a broadband light source (emission between 200 and 1000 nm) and a receiver-spectrometer assembly. The spectral data from the spectrometer are processed in real time using a PC computer to calculate the concentrations of the pollutants programmed to be monitored by the system. The distance between the light source and receiver can range from 100 m to 2,000 m, depending on the pollutant to be monitored and species concentrations. In September and October of 1989, an OPSIS AB DOAS was operated in the Research Triangle Park, NC on the roofs of the two main EPA laboratories.