Photocatalytic pretreatment of waste activated sludge (WAS) using a flat photocatalytic reactor was undertaken. Photocatalytic pretreatment enhanced the release of soluble substances from WAS, in which the soluble protein and soluble carbohydrate concentration increased by about 50% and 80%, respectively. Significant removal of heavy metal ions from the liquid phase of WAS was also achieved after photocatalytic pretreatment. In addition, the highest hydrogen yield and the highest concentration of volatile fatty acids (VFAs) were achieved from the photocatalysis pretreated WAS by batch anaerobic digestion (55 degrees C). The cumulative hydrogen yield from photocatalysis pretreated WAS was 211.0 ml/l-sludge, much higher than those from UV pretreated WAS (111.0 ml/l-sludge) and from raw WAS (93.0 ml/l-sludge). The results indicate that photocatalysis is a promising WAS pretreatment method for the enhancement of biohydrogen production, probably due to the photo-oxidation of organics and simultaneous photo-reduction of heavy metal ions in WAS. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.