A method reducing dioxins in combustion installations having at least one deposition surface, wherein the deposits of carbon-rich ash from the flue gas are reduced to short time intervals, thus reducing the formation of PCDD/F. The object is to propose a method for on-line determination of the carbon concentration of deposited ash, in particular with considerably improved spatial and time resolution of the ash parameters which govern the dioxin formation rate. The object is achieved by a method comprising – optical detection of the ash deposits on at least one measurement point on the deposition surfaces by means of a video camera in the visible wavelength range, - determination of the reflectivity (9) of the surface of the ash deposits at the measurement points – conversion of the reflectivity to a corresponding TOC value (14, 15), and initiation of cleaning of the ash deposits when a TOC threshold value (11) is exceeded by the TOC value (15).