Microbial population dynamics and anaerobic digestion (AD) process to eight different hydraulic retention times (HRTs) (from 25 d to 3.5 d) in two-phase dry-thermophilic AD from sulphate-containing solid waste were investigated. Maximum values of gas production (1.9 +/- 0.2 l H-2/l/d; 5.4 +/- 0.3 l CH4/l/d and 82 +/- 9 ml H2S/l/d) and microbial activities were obtained at 4.5 d HRT; where basically comprised hydrolysis step in the first phase (HRT = 1.5 d) and acidogenic step finished in the second phase as well as acetogenic-methanogenic steps (HRT = 3 d). In the first phase, hydrolytic-acidogenic bacteria (HABs) was the main group (44-77%) and Archaea, acetogens and sulphate-reducing bacteria (SRBs) contents were not significant; in the second phase (except to 2 d HRT), microbial population was able to adapt to change in substrate and HRTs to ensure the proper functioning of the system and both acetogens and Archaea were dominated over SRBs. Decreasing HRT resulted in an increase in microbial activities. (C) 2013 Elsevier Ltd. All rights reserved.