tThe extent of syntrophic acetate oxidation (SAO) and the levels of known SAO bacteria and acetate- andhydrogen-consuming methanogens were determined in sludge from 13 commercial biogas productionplants. Results from these measurements were statistically related to the prevailing operating condi-tions, through partial least squares (PLS) analysis. This revealed that high abundance of microorganismsinvolved in SAO was positively correlated with relatively low abundance of aceticlastic methanogensand high concentrations of free ammonia (>160 mg/L) and volatile fatty acids (VFA). Temperature wasidentified as another influencing factor for the population structure of the syntrophic acetate oxidisingbacteria (SAOB). Overall, there was a high abundance of SAOB in the different digesters despite differ-ences in their operating parameters, indicating that SAOB are an enduring and important component ofbiogas-producing consortia.