The practical application of most of the UV-based advanced oxidation/reduction technologies (AOTs) is still limited because of the poor selectivity of the reactive species in the UV-based AOTs and the inner filter effects caused by the complicated components in real waters. Recently, new photochemical techniques based on low molecular weight diketones have attracted much attention because of the higher selectivity and the stronger anti-interference ability. This review summarizes the current research progress on diketone-mediated photochemical processes, with emphasis on the role of the enol tautomer in electron/energy transfer. The characteristics of UV/diketone and its potential applications in water pollution control are preliminarily discussed. Moreover, the remaining questions in the current research on UV/diketones are analyzed, and the future research directions are proposed.