The spectra and relative efficiency of200Hg 79Br, Hg 79Br, and HgBr in a small UV preionized electric discharge laser have been measured. There is a similar improvement in laser energy for both isotopic samples compared to the natural abundance mixture which can be attributed to a narrower gain spectrum and consequent higher peak gain. At the highest output measured, the increase in laser energy was about 25%. We estimate this is due to roughly 15% increase in small‐signal gain.