Biological sulfate reduction was studied in laboratory-scale gas-lift reactors, Synthesis gas (gas mixtures of H-2/CO/CO2) was used as energy and carbon source. The required biomass retention was obtained by aggregation and immobilization on pumice particles. Special attention was paid to the effect of CO addition on the sulfate conversion rate, aggregation, and aggregate composition. Addition of 5% CO negatively affected the overall sulfate conversion rate; i.e., it dropped from 12-14 to 6-8 g SO42-/L . day. However, a further increase of CO to 10 and 20% did not further deteriorate the process. With external biomass recycling the sulfate conversion rate could be improved to 10 g SO42-/L . day. Therefore biomass retention clearly could be regarded as the rate-limiting step. Furthermore, CO affected the aggregate shape and diameter. Scanning electron microscopy (SEM) photographs showed that rough aggregates pregrown on H-2/CO2 changed into smooth aggregates upon addition of CO. Addition of CO also changed the aggregate Sauter mean diameter (d(32)) from 1.7 mm at 5% CO to 2.1 mm at 20% CO. After addition of CO, a layered biomass structure developed. Acetobacterium sp. were mainly located at the outside of the aggregates, whereas Desulfovibrio sp. were located inside the aggregates.