- The loss of O 2 ( 1 Δ) during iodine dissociation in the chemical oxygen-iodine laser (COIL) is one of the main factors affecting the output power. Analytical expression is obtained for the number of O 2 ( 1 Δ) molecules, N, lost in the region of I 2 dissociation per one molecule of I 2 . This expression yields N = 4-6, in agreement with numerical calculations and experimental measurements. It is shown that some effective number N 1 < N should be used rather than N to calculate the power. Analytical expression for the power is obtained, taking into account the O 2 ( 1 Δ) losses in the dissociation region. It is shown that N increases and the power decreases when the dissociation fraction F increases. Therefore, maximum power is achieved at low values of the iodine flow rate when iodine is not completely dissociated before the resonator and when the small signal gain in the resonator region is less than its maximum achievable value. Numerical modeling of the RADICL (supersonic COIL) is carried out. The values of the O 2 ( 1 Δ) yield and of the mixing rate are estimated to reach an agreement between the calculated and the measured dependencies of the power on the iodine flow rate.