The impact of stepwise increase in OLR (up to 7.5 kgVS/m(3) d) on methane production, reactor performance and solubilised organic matter production in a high-loading reactor were investigated. A reference reactor operated at low OLR (<2.0 kgVS/m(3) d) was used solely to observe the methane potential of the feed substrate. Specific methane yield was 0.331 CH4/gVS at the lowest OLR and dropped by about 20% at the maximum OLR, while volumetric methane production increased from 0.35 to 1.38 m(3)CH(4)/m(3) d. At higher loadings, solids hydrolysis was affected, with consequent transfer of poorly-degraded organic material into the drain solids. Biodegradability and size-fractionation of the solubilised COD were characterized to evaluate the possibility of a second stage liquid reactor. Only 18% of the organics were truly soluble (<1 kD). The rest were in colloidal and very fine particulate form which originated from grass and cow manure and were non-biodegradable. (C) 2012 Elsevier Ltd. All rights reserved.