Monocot residues such as corn stover and straw are often not fully exploited and constitute a potential substrate for bioethanol production. However, a number of factors such as high enzyme loadings make large-scale utilization economically difficult. Addition of non-ionic surfactants and poly(ethylene glycol) to enzymatic hydrolysis of various lignocellulosic substrates has been found to increase the conversion of cellulose into soluble, fermentable sugars. We have shown that surfactants are able to increase cellulose conversion with up to 70%. This provides an opportunity of decreasing enzyme loading while retaining the same degree of hydrolysis. Investigations of five wheat straw substrates produced with different pretreatment methods revealed that surfactants have a more pronounced effect on acid and steam treated straw than, e.g. ammonia and hydrogen peroxide treated straw. Thus, lignin content is not directly proportional with the potential surfactant effect. Studies of adsorption of cellulases support the theory that the main mechanism behind the surfactant effect is prevention of unspecific adsorption of enzyme on the substrate lignin. This is believed to be due to hydrophobic interaction between lignin and the surfactant, causing steric repulsion of enzyme from the lignin surface. More research is needed to reveal which factors influence enzyme and surfactant adsorption. (c) 2006 Elsevier Inc. All rights reserved.