Abstract: The present invention relates to an apparatus and method for determining the concentration of nitric oxide (NO) in a gas mixture such as air. The gas sample containing NO is mixed with a gas containing ozone (O3), and the change in the ozone concentration is measured after a sufficient time is allowed for the reaction between NO and O3 to take place and destroy a measurable quantity of O3. In the disclosed embodiment, the concentration of ozone is measured using the technique of UV absorption. In this case, the invention has the advantage over other instruments for measuring NO of having absolute calibration based on the known extinction coefficient for ozone at ultraviolet wavelengths. The invention discloses both static and dynamic flow systems, and the NO concentration measurements may be made over a wide pressure range.