HgH and HgD were detected in absorption in the mercury flash-photosensitized reactions of Hg(3P1) and Hg(3P0) with H2 and D2. By comparing the transient decrease of the vapour pressure of mercury in a flashed mixture of Hg and H2 with that in a mixture of Hg and N2, approximate quantum yields for mercury hydride formation were measured. HgH was also detected in a flashed mixture of Hg and C2H2. Mercury hydride was not detected in the quenching of Hg(3P1) by saturated hydrocarbons, HCN, NH3, H2O, HCl, HBr and H2S.