The influence of sodium hydroxide addition during the thermo-chemical pretreatment of a microbial biomass has been evaluated in terms of particulate chemical oxygen demand (COD) solubilization and anaerobic biodesadability. Both COD solubilization and total solid elimination rates increased with the dose of NaOH added: COD solubilization reached 63%, and total solid elimination was 33% when 5 g Na/l were added. Additional sodium hydroxide addition did not increase COD solubilization further. The observed COD solubilization was due mainly to protein hydrolysis that was directly linked to pH variations. Heating emphasized these pH effects. The higher sodium hydroxide addition (26.1 g/l) led to 85% COD solubilization when heated to 140 degrees C for 30 min instead of 53.2% which was at ambient temperature. Biodegradability and biotoxicity tests were run with pretreated samples. Both methane production and acetate degradation in the presence of pretreated samples were affected as 5 g NaOH/l or more were added. Sodium cation was first suspected to account for the limitations observed. Tests run at pH = 12 with other alkali agents (KOH, Mg(OH)(2), or Ca(OH)(2)), led to the same conclusion: COD solubilization was enhanced but the biodegradability performances were limited. Additional tests run with acetate and increasing NaCl concentrations asserted that sodium cation was not at the origin of the limitations that were attributed to OH- additions: refractory compounds were formed as a consequence of pH modifications.