The feasibility of remotely monitoring the concentration of nitric oxide (NO) in the effluent of industrial stacks has been investigated analytically and experimentally in the laboratory. The type of instrument considered employs two or more gas-filter cells that contain different amounts of NO. Radiant energy emitted by the hot gas in the effluent is measured after it has passed either through one of the gas-filter cells or through a neutral density filter. By comparing the amounts of energy received through each of the filters, it is possible to determine the concentration of NO in the presence of a moderate amount of continuum-emitting material such as small particles. A simple, single-line spectral model served as the basis for the analytical work. Heated cells containing NO + N2 or H2O + N2 simulated an industrial stack for the laboratory experiments. Interference by hot H2O in the effluent and cold H2O in the atmospheric path causes the most serious uncertainties in the measurements for many types of stacks.