An instrument has been developed to continuously measure the steam content in flowing steam/air mixtures. This instrument is nonintrusive and thus can be used in existing and proposed future experimental facilities in such a way as to not affect the flow field. The instrument detects the steam content by measuring the differential attenuation of infrared radiation passing through the flowing fluid. A small steam/air test facility has been designed and constructed and was used to calibrate this instrument. Experiments have indicated an instrument accuracy of ±5% in the measurement of steam/air fraction. This is particularly impressive considering the small (1.9-cm) steam/air attenuation path utilized.