A gas mixture of a gas discharge laser such as an excimer or molecular fluorine laser is stabilized. The gas mixture of the laser includes a constituent halogen containing molecular species such as F2 or HCl which is subject to depletion from an initial optimum concentration. When the gas mixture is energized by a pulsed discharge circuit, the amplified spontaneous emission (ASE) signal is monitored. The status of the gas mixture is determined based on the monitored ASE signal. Stimulated emission is preferably filtered or blocked for more precise ASE signal monitoring. The gas mixture is preferably replenished using small halogen injections, total pressure adjustments and mini and partial gas replacements based on the evolving gas mixture status determined from the monitored ASE signal. Another parameter may be monitored such as the driving voltage to indicate the status of another system specification such as alignment or degradation of resonator optics, synchronization of optical components, contamination within the resonator or purge conditions for a F2-laser enclosure.