A study has been made of the reaction of Hg2026(3P1) atoms, photoexcited in natural mercury vapor; with a number of substrates which form solid mercury compounds in mercury photo-sensitization. Some data are also given for reactions initiated by Hg1986(3P1) atoms. The solid mercury compounds formed were examined for enrichment in the isotope initiating the reaction. Such enrichment would be evidence for the primary formation of the mercury compound.Three HgO-forming substrates were studied: water vapor, nitrous oxide, and oxygen. The Hg2Cl2-forming substrates studied included hydrogen chloride, methyl chloride, methylene chloride, chloroform, carbon tetrachloride, isopropyl chloride, and boron trichloride. One Hg2Br2-forming substrate was examined—isopropyl bromide.Among the HgO-forming substrates only water vapor gave enrichment in Hg202in the HgO product. The Hg202content of the oxide was found to vary from 32 to 35%, depending on reaction conditions, compared to a natural abundance of 29.6%. With water vapor – butadiene mixtures, oxides containing as high as 90% Hg202were obtained. Similar enrichment factors were obtained for Hg1986(3P1) reactions.Hydrogen chloride and the alkyl chlorides yielded calomels containing a maximum of 45% Hg202. Methyl chloride gave similar enrichment factors in Hg198for the Hg1986(3P1) reaction. The calomel formed in the boron trichloride reaction showed no enrichment.Addends such as butadiene and benzene, when added to hydrogen chloride, increased the Hg202enrichment from 45% to 60%.For those reactions which yield mercury compounds enriched in the initiating mercury isotope, evidence is presented for a single primary process. Failure to obtain pure isotopes is attributed to exchange reactions with adsorbed natural mercury during recovery of the enriched mercury from the product. It is postulated that the addends react with the mercury product and reduce chemisorption of natural mercury on the product.The significance of these findings in the mechanisms of the reactions studied is discussed.