Microbiological aspects of biological sulphate reduction in gas-lift reactors were studied. Hydrogen and carbon dioxide were used as energy and carbon sources. Biomass retention was obtained by aggregation and natural immobilization on pumice particles. Biological sulphate reduction on H-2/CO2 appeared to be applicable within a pH range of 5.5-8.0 with an optimum near pH 7.5. The pH affected aggregate configuration and diameter At pH 7.0, the average Sauter mean diameter of the aggregates was 1.5 mm. Moreover; phase-contrast and SEM microscopy showed highly branched aggregate surfaces. A pH increase led to increased surface irregularity without affecting the particle diameter A pH decrease caused a decreased surface irregularity and changed the aggregate Sauter mean diameter from 1.50 mm at pH 7.0 to 2.26 at pH 5.5. However the pH did not have a significant effect on the biomass composition. Examination of the bacterial composition of the aggregates by phase-contrast microscopy, SEM microscopy, as well as enrichments, showed that at all pH values Desulfovibrio species and Acetobacterium species were the most abundant micro-organisms.