An investigation has been made of the reaction of Hg202(3P1) atoms, photoexcited in natural mercury vapor, with several HgO-forming substrates, including water vapor, nitrous oxide, and oxygen. The reactions were carried out under flow conditions at room temperature. Emphasis has been placed on the Hg202content of the HgO product. Enrichment in Hg202would be evidence of the primary formation of a mercury compound.In the reactions with nitrous oxide and oxygen, it was found that the HgO product contained the normal abundance of Hg202. With water vapor, a product containing up to 35% Hg202was obtained, compared to the normal abundance of 29.8%. The addition of 1,3-butadiene to the water vapor substrate was found to have a marked effect on the enrichment in Hg202in the HgO product. A product containing 85% Hg202was recovered when the substrate contained 21 mole % of added butadiene. A systematic study of the effect of butadiene concentration on the enrichment reaction showed that the Hg202content remains constant for butadiene concentrations exceeding approximately 10 mole %. The water–butadiene reaction is recommended as a simple method for preparing Hg202from natural mercury.The factors influencing the enrichment process are discussed, together with the significance of the results with respect to the mechanisms of decomposition of the molecules studied.