Experimental results of small‐signal gain in CO chemical laser, operating with very high values of CS:CS2and S:CS, are presented. The laser is produced in a shock tunnel where complete thermal dissociation of CS2into CS+S is accomplished by a reflected shock wave. The dissociated products, diluted in Ar, expand through a supersonic nozzle into a combustion chamber, where molecular oxygen is supersonically injected. The two streams react, and vibrationally excited CO is produced. The experimental data are compared with a numerical model developed for this laser. A fairly good agreement between calculations and experiments was found, which enables the utilization of the mathematical model for better understanding of the processes involved in laser operations. Gains for P7(14) and P5(18) were measured at distances of 5.5 and 11.5 cm downstream from oxygen injectors. Stagnation pressure of 25 atm was kept constant in all experiments. Stagnation temperatures were varied in the range of 2500–4600 °K, and mixtures of CS2:Ar between 5:90 and 30:70 were tested. A maximum of small‐signal gain was observed for each of the parameters investigated. The maximum gain measured for P7(14) was 8% cm−1for CS2:Ar = 20:80 and stagnation temperature of 4000 °K. For these operating conditions, laser oscillations were observed in an 8‐cm‐long active cavity. The resonator consists of a full reflector and a 42% reflective coupling mirror.