The invention refers to methods and compositions for stabilizing and increasing the activity of enzymatic mixtures comprising GH61 (PMO or polysaccharide monooxigenase) polypeptides used for the degradation of cellulosic material during the saccharification step of biofuel production processes. This improvement is achieved by the addition of a nickel cation to said enzymatic mixtures before and/or during the saccharification step. Thus, the invention provides compositions comprising PMOs, cellulolytic enzymes and a nickel cation, as well as methods for preparing said compositions and methods for producing fermentable sugars and bioproducts, preferably bioethanol, from cellulosic biomass in which said compositions are used.
