- Measurements of the gain and temperature in the resonator of a supersonic chemical oxygen–iodine laser (COIL) with transonic injection of I 2, using diode laser-based diagnostics, are reported. For conditions corresponding to the maximum chemical efficiency of this type of COIL (11.7 mmole/s of chlorine, no primary buffer gas, and 1.36 mmole/s of the secondary N 2), the gain is a nonmonotonous function of the iodine flow rate. Maximum gain of 0.34%/cm is achieved for an iodine flow rate of 0.27 mmole/s. An analytical method is developed which enables the use of these dependencies for calculation of the iodine dissociation fraction F and the number N of O 2 (1 Δ) molecules lost in the region of iodine dissociation per I 2 molecule. It is found that F is a nonmonotonous function of the iodine flow rate, its maximum value being small (0.55).