The effect of sludge disintegration on molecular weight distribution of dissolved organic carbon (DOC) is demonstrated by size exclusion chromatography with online organic carbon detection (SEC-OCD). Ozone, ultrasound and sodium hydroxide were used to disintegrate excess sludge. Significant differences on the molecular weight distribution of the released DOC were found for the different treatments. Ozonation did not change the DOC distribution based on the molecular size, compared to untreated sludge. Small molecular weight compounds were the dominant constituents solubilized by sodium hydroxide treatment. Ultrasound disintegrated the sludge flocs and dissolved large molecules into the sludge liquid phase. Especially for sodium hydroxide and ultrasound treatment the analysis with SEC-OCD clearly shows the preferential fractions of organic carbon, which are more quickly transferred to methane in batch anaerobic experiments. For sodium hydroxide, it is the small molecular weight fraction as for ultrasound treatment the degradation is evenly distributed throughout all weight fractions.