Yellow coloration of TiO 2 (anatase) was observed on uv irradiation in the presence of Hg vapor and O 2. The effect was most pronounced with light of 390–420 nm, corresponding to the absorption edge of TiO 2. At room temperature, the yellow color darkened with continued irradiation, and finally a dark brown surface layer was formed in which HgO was identified. At 170 °C, only yellow coloration of the TiO 2 was observed, and there was a saturation in Hg content corresponding to the quantity of acidic as well as of basic OH groups on the surface. A grey instead of a yellow tone appeared with TiO 2 the basic OH − groups of which had been exchanged for H 2PO 4 − ions. Hg of oxidation state +1, exclusively, was found on phosphate-covered TiO 2. Mercury(I) was found also in the beginning of the photooxidation reaction on untreated or alkali-treated TiO 2; later on, there was only Hg 2+ present. One O 2 molecule is consumed for each chemisorbed Hg atom at 60 °C and at low uptakes. Water is formed in this reaction. The photooxidation is apparently caused by OH radicals, arising from the basic OH − groups, together with a chemisorbed oxygen species, very likely O 2 −. The reaction is reversible: the yellow color fades slowly in darkness if water vapor is present, and metallic Hg distills off. The same reactions were observed qualitatively with rutile as well as with anatase, and with zinc oxide, tin dioxide, or cerium dioxide.