A study was undertaken to investigate controlled mesophilic anaerobic digestion of both the supernatant and the sludge from settled olive-oil wastewater. Two different types of anaerobic digesters were used, a fixed-bed type for the supernatant and a plug-flow type for the sludge. Concentrated aqueous ammonia and sodium carbonate were added to adjust the CIN ratio and the pH of the solution in each digester. In both digesters, biogas production and chemical oxygen demand (COD) reduction exceeded the rates mentioned in the literature for diluted raw olive-oil wastewaters. In the digester fed with the supernatant, biogas production rate was stabilised at a mean value of 1.86 litres/litre working volume (average of 24 biogas values) with a COD reduction of 90.91 %. In the digester fed with the settled sludge, biogas production rate was stabilised at a mean value of 2.75 litres/litre working volume (average of 12 biogas values) with a COD reduction of 94.95%. These values expressed for the total olive-oil wastewater resulted in an overall biogas production of 2.28 litres/litre working volume and in an overall COD reduction of 94.02% with a final COD concentration of 4000 mg/litre olive-oil wastewater. The results indicated that a total reactor volume of 9.2 litres/litre wastewater was required, 4.08 litres for the fixed-bed and 5.12 litres for the plug-flow digesters. In this way, the anaerobic-digestion system suggested is of reduced size and becomes cost-effective compared with other digestion systems suggested in the literature for such wastes.