Material balances of volatile solids, total dry solids and COD in an anaerobic digester fed with mixtures of domestic sludges and prehydrolyzed lignocellulosics are proposed to describe fed-batch operation. Separate complete prehydrolysis of the feed performed to increase its contents of soluble organic substances proves to strongly affect the methanogenic phase, probably due to the formation of toxic inhibitors during lignin hydrolysis. This suggests that the removal of the residue remaining after hemicellulosic sugars solubilization is necessary to offer acceptable methane yields in a poorly-mixed digester as that used in this study. Although the proposed approach seems to apply satisfactorily to COD only at relatively low organic load, it can be proposed as a useful tool to follow the solids breakdown during fed-batch operation.