Solids concentration is a critical technico-economic parameter in biohydrometallurgical processing since it decisively affects bioreactor size and therefore investment and operating costs. Researchers have repeatedly provided evidence, since the early times of biohydrometallurgy, that bioreactor leaching rates of metal sulphide concentrates drastically decrease whenever percent solids concentration exceeds 16-20. This limitation, should it hold also for coal biodepyritization processes, would place a serious constraint on their economics. The paper presents the results of an investigation carried out running in parallel three different types of laboratory reactors fed with the same pulp processed in the 7.5 m(3) stirred-tank reactors (STRs) installed in the Porto Torres coal biodepyritization pilot plant. Evidence is provided that biodepyritization performance of the reactors remains practically unchanged in the solids concentration range from 5% to 40%, whereas failure of pure pyrite bioleaching at 30% solids concentration is observed in the same reactors. This result seems to support the view that solids concentration influence on bioleaching performance is related to the proportion of oxidizable mineral present in the solid phase. Comparison of laboratory- and semi-commercial-scale STRs seems to show that scale-up imparts an improvement in machine performance.