A repetitively pulsed iodine laser incorporating a closed cycle flowing gas system has been developed in which the reversibility of photodissociation has been studied for both free-running oscillator and amplifier modes. Consumption of the working material is a significant limitation on the use of the iodine laser as a high average power, source, and we have achieved some improvement in the amplifier mode by introducing excited state quenching agents. A kinetic model gives good agreement with pulsed oscillator behaviour, and has been extended to describe a 30 watt CW flowing system. Improved photochemical reversibility and an efficiency of 1.5%.are predicted.