The development of environmental catalysts is an urgent subject to be tackled by scientists and engineers all over the world due to the borderless nature of environmental pollution. We named the catalyst enabling the decomposition of the pollutants by effectively utilizing the solar energy from ultraviolet to infrared as “solar environmental catalysts”. This Feature Article reviews the recent studies on a novel class of solar environmental catalysts consisting of TiO2 and molecular scale oxides of 3d metals and for comparison d10 (Sn) on the surface (MOs/TiO2). The TiO2 surface modification with MO clusters by the chemisorption–calcination cycle (CCC) technique presents novel band engineering for finely tuning the top of the valence band, while the unique physicochemical and electronic properties of MOs/TiO2 give rise to the outstanding photo- and thermocatalytic activities for the decomposition of organic pollutants. In the first part following the Introduction, the CCC technique for forming extremely small MO clusters on TiO2, the structures, physicochemical properties, and electronic structures of MOs/TiO2 are described. The second part deals with their thermo- and photocatalytic activities for the degradation of model organic pollutants and the essential action mechanisms of the MO clusters. The combination of experiments and first-principles density functional theory simulations shows that Co2O3/TiO2 can be a prototype of the solar environmental catalyst with high levels of photo­(UV and visible)- and thermocatalytic activities.