External carbon sources such as methanol and acetate are applied in order to improve the denitrification efficiency in biological nitrogen removal processes. Excess sludge wasted from the treatment plants contains organic carbon that could be used as a feasible carbon source for denitrification. However, the excess sludge needs to be hydrolyzed first before being utilized as a carbon source. The objective of this research is to evaluate the biodegradability of the sludge hydrolysate obtained using alkaline, acid, autoclaved and combined solubilization methods. The experimental results suggest that the method of solubilization has an impact on the biodegradability of sludge hydrolysate. Autoclaved alkaline sludge hydrolysate gave the fastest denitrification rate (250 mgN/gMLSSCOD/d) due to fastest protein degradation (247 mgCOD/gLSSCOD/d) and other COD degradation rates (370 mgN/gMLSSCOD/d). Alkaline, autoclaved and acid autoclaved sludge hydrolysates gave nearly the same denitrification rates (185 to 200 mgN/gMLSSCOD/d). Protein degradation rate was the highest for autoclaved alkaline sludge hydrolysate (247 mgCOD/gMLSSCOD/d) whereas carbohydrate degradation rate was the fastest for autoclaved acid sludge hydrolysate (91 mgCOD/gMLSSCOD/d). For the same enzyme activity, different carbohydrate and protein degradation activities have been found suggesting the presence of easily and slowly biodegradable portions in them.