This study evaluated hydrogen production and chemical oxygen demand removal (COD removal) from tapioca wastewater using anaerobic mixed cultures in anaerobic baffled reactor (ABR). The ABR was conducted based on the optimum condition obtained from the batch experiment, i.e. 2.25 g/L of FeSO4 and initial pH of 9.0. The effects of the varying hydraulic retention times (HRT: 24, 18, 12, 6 and 3 h) on hydrogen production and COD removal in a continuous ABR were operated at room temperature (32.3 +/- 1.5 degrees C). Hydrogen production rate (HPR) increased with a reduction in HRT i.e. from 164.45 +/- 4.14 mL H-2/L.d (24 h HRT) to 883.19 +/- 7.89 mL H-2/L.d (6 h HRT) then decreased to 748.54 +/- 13.84 mL H-2/L.d (3 h HRT). COD removal increased with reduction in HRT i.e. from 14.02 +/- 0.58% (24 h HRT) to 29.30 +/- 0.84% (6 h HRT) then decreased to 21.97 +/- 0.94% (3 h HRT). HRT of 6 h was the optimum condition for ABR operation as indicated. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.