This paper reports on the influence of the liquid-phase mass transfer on the performance of a horizontal-flow, anaerobic, immobilized-biomass (HAIB) reactor treating low-strength wastewater. The HAIB reactor was subjected to liquid superficial velocities (vs) ranging from 10 to , corresponding to hydraulic detention time (θh) of 10–2 h. The best performance was achieved at an overall θh of 3.3 h due to the interdependence of biochemical reactions and mass transfer mechanisms for process optimization. The HAIB reactor was provided with four intermediate sampling ports, and the values of vs were fixed to permit sampling at different ports corresponding to θh of 2 h as vs increased. The chemical oxygen demand removal (COD) efficiencies increased from 68% to 82% with the increase of vs from 10 to . It could be concluded that the performance of the HAIB reactor was improved significantly by increasing vs, thus decreasing the liquid-phase mass transfer resistance.