The main objective of this work has been to analyze the effect of hydraulic retention time (HRT) on the hydrogen production from the organic fraction of municipal solid waste (OFMSW) coming from a full-scale mechanical-biological-treatment (MBT) plant. Furthermore, it has also been studied, simultaneously, the effect of HRT on the solubilization of organic matter. Experiments were conducted in an anaerobic continuous stirred tank reactor (CSTR) operating at thermophilic-dry conditions (55 degrees C and 20% in total solids concentration respectively). A decreasing sequence of nine HRTs, from 15 days to 1.5 days, was imposed to evaluate its influence on the hydrogen production (HP), the specific hydrogen production (SHP) and the solubilized organic matter expressed in form of acidogenic substrate as carbon (ASC), dissolved organic carbon (DOC) and dissolved acid carbon (DAC). The results have shown that the best results were obtained at 1.9-day HRT with a feeding regime of twice a day (type-C). At these conditions, the HP and the SHP was 1.077 L H-2/L-reactor day and 24.3 mL H-2/g VSaddech respectively. Maximum concentrations obtained of solubilized organic matter were: 1201 mg/L for ASC, 1936 mg/L for DOC and 735 mg/L for DAC. As novelty, the parameter ASC and, specially, the ratio ASC/DOC have shown to be adequate for analyzing and interpreting the behavior of the process and, concretely, to determine if hydrolysis and acidogenesis are coupled (stable process) or decoupled (transitory stage). (C) 2013 Elsevier B.V. All rights reserved.