Measurements of the small-signal gain of the laser medium produced by a combustion-driven CW chemical laser are presented. The laser characterized is a conventional supersonic flow chemical laser. A low-power CW chemical probe laser operating on a single line and single TEM00qmode was used for the measurements. Scans of the cavity in the streamwise direction were made by directing the probe beam through the cavity with a set of translating mirrors. We present a description of the probe laser, scanning mechanism, and signal processing system which gave good repeatability and excellent signal-to-noise performance. Streamwise scans of the laser cavity at two nozzle heights are presented. One of the scans was made at the nozzle center, and the other near the nozzle edge where free-jet expansion affects the gain medium. The measured gain profiles obtained at these two heights are compared and analyzed for several laser lines and several flow conditions. Finally, we analyze these data in terms of measured laser performance parameters.