This paper reports the effects of illumination on methanogenic performance using acetate synthetic wastewater as the organic substrate. Several different illumination times such as 0 (continuous darkness), 15, 30, 45, 60, 90,150, 240 and 480 min/day and continuous light were examined in a laboratory set-up under batch conditions. The reactors, when exposed to short illumination times such as 60 min/day, showed the greatest methane concentration, methane yield, and dissolved organic carbon (DOC) removal, which were 78.5%, 176 ml/g-VS, and 75.8%, respectively, in comparison to other conditions. Furthermore, the performances of three fluidized bed reactors operated in semi-continuous mode under conditions of continuous darkness, 60 min/day illumination, and continuous illumination were investigated. The methane yield and DOC removal was 794 ml/g-DOC added, 893 ml/g-DOC added, and 596 ml/g-DOC added, and 88.3%, 93.7%, and 60.5%, respectively. in the present investigation, illumination for 60 min/day for microbial treatment was found to be more effective than either continuous darkness or continuous illumination. Molecular and microscopic study revealed that the major methanogens in the bed reactors belonged to the genus Methanosarcina sp. Immobilized cells from the short-time illumination and continuous darkness reactors show a smooth surface membrane. However, an uneven surface membrane and destruction of cell tissue were observed in the reactor with continuous illumination. (C) 2008 Elsevier B.V. All rights reserved.