The stability of the high pressure discharge excited XeCl excimer laser are related to the concentration of halogen donor HC1. In this paper, a theoretical analysis of the equation for continuity of the electrons in the discharge plasma predicate that the duration of the uniform phase should vary as the inverse square root of the HC1 concentration. This prediction is in good agreement with experimental results obtained from a X-ray pre-ionized PFN pumped XeC1 excimer laser.