The invention relates to a reactor for carrying out photocatalysed reactions in liquid or gaseous reaction media, consisting of a reactor vessel with a solid photocatalyst (PC), feed lines and take-off lines, mixing means, and a means of supplying electromagnetic radiation, containing microradiators which absorb the electromagnetic radiation and, with a time delay, emit light which excites the photocatalyst, and also to a process for carrying out photocatalytic reactions, in which solid PC are suspended in the liquid or gaseous reaction medium and are activated by means of microradiators which are charged up at an electromagnetic radiation source and which emit this energy with a time delay.