Solid retention time (SRT) is a very important operational variable in continuous and semicontinuous waste treatment processes since the organic matter removal efficiency - expressed in terms of percentage of Dissolved Organic Carbon (% DOC) or Volatile Solids (% VS) removed - and the biogas or methane production are closely related with the SRT imposed. Optimum SRT is depending on the waste characteristics and the microorganisms involved in the process and, hence, it should be determined specifically in each case. In this work a series of experiments were carried out to determine the effect of SRT. from 40 to 8 days, on the performance of the dry (30% Total Solids) thermophilic (55 degrees C) anaerobic digestion of organic fraction of Municipal Solid Wastes (OFMSW) operating at semicontinuous regime of feeding. The experimental results show than 15 days is the optimum SRT (the best between all proved) for this process. Besides, data of organic matter concentration and methane production versus SRT have been used to obtain the kinetic parameters of the kinetic model of Romero Garcia (1991): the maximum specific growth rate of the microorganisms (mu(max) = 0.580 days(-1)) and the fraction of substrate non-biodegradable (alpha = 0.268). (C) 2010 Elsevier Ltd. All rights reserved.