Dry mesophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is a widespread technology. However, OFMSW is a very heterogeneous waste and therefore specific studies are needed to optimise the AD process with each type of OFMSW that will be used. The main operational variables to optimise are the solids retention time (SRT) and the organic loading rate (OLR), which are closely related to each other when the concentration of organic matter of OFMSW remains relatively constant. In this work three different experimental conditions (corresponding to the SRT of 30, 20 and 15 days) were tested in a semi-continuous stirred tank reactor, operating at mesophilic range (35 degrees C) and high solids concentration (20% TS). The OLR corresponding to the above mentioned SRT were 22.8, 27.3 and 35.9 mg DOC/L-reactor/d, respectively. The results obtained in this study indicate that 20 days is the optimum SRT for the dry mesophilic anaerobic digestion of the OFMSW used. Thus, in general, all the parameters analysed show better performance for 20-day SRT with regard to 30-day and 15-day SRT. More specifically, it can be pointed out than at SRT of 20 days (27.3 mg DOC/L-reactor/d), both the highest productivity of methane (0.11 L CH4/g waste-fed) and the highest organic matter removal rate (66.3% DOC removal) were reached. (C) 2012 Elsevier B.V. All rights reserved.