Transient absorption in Ne/Xe/HCl mixtures and various rare gas combinations are measured. The observed results are matched with a numerical model to determine the dominant absorbing species in the XeCl laser. In pure neon plasmas Ne*2is a dominant absorber. Addition of xenon greatly reduces this absorption through the mechanism of Penning ionization. Absorption is not changed by adding HCl to the optimum neon‐xenon concentration.