In the Hg-sensitized photolysis of H-2 in the presence of C2H4 the sensitizer Hg is consumed under formation of a metastable compound. The rate of formation and the steady-state concentration of this compound have been studied as a function of Hg, H-2, and C2H4 concentration, absorbed light intensity, temperature, and in the presence of radical scavengers. All these experiments ask for the participation of HgH and C2H5 in the formation of the Hg compound. We therefore suggest that these two radicals combine under formation of HHgC2H5. Further support comes from two color photolysis experiments where HgH and C2H5 radicals have been generated independently. HHgC2H5 decomposes under 206 nm irradiation and much slower in the dark. It is also very easily attacked by radicals.