1. The complete difference spectrum of the reaction of the photoactive chlorophyll-an is presented. 2. The reaction of excited chlorophyll-aII is of the type of a sensitizer. It is not engaged directly in the electron transfers. This is in contrast to the photoactive chlorophyll-aI which is an electron donor in its excited state. 3. The chlorophyll-aII-reaction can be separated from the overall reaction by heating chloroplasts 5 min at 50°C. 4. Chlorophyll-an is the reaction center of the well-known poison DCMU. 5. Properties of chlorophyll-aII are depicted in Tab. 1. They are compared with those of chlorophyll-aI and the O2-evolution system.