Cereal straw is a large bioresource but requires pretreatment before use in biogas production. We investigated ensiling of wet straw as a biological pretreatment for enhancing the biochemical methane potential (BMP). Lab-scale ensiling experiments with wheat and barley straw showed that pH reduction during ensiling was enhanced by high water content (up to 50 %), chopping and the ensiling additives lactic acid bacteria, formic acid, acetic acid and brown juice. Compared to dry, non-ensiled wheat straw, BMP was significantly increased after ensiling, with up to 19, 24 and 24 % increase after three, six and ten months of ensiling, respectively. BMP increased significantly with increasing water content in wheat straw during ensiling. There was only a limited additional effect on BMP from chopping and ensiling additives. For barley straw, BMP was increased up to 28 % after ensiling for six months. The results demonstrate ensiling of wet straw as a potential pretreatment method.