The short-chain fatty acids (SCFAs) accumulated in waste activated sludge (WAS) fermentation was adopted as an alternative extra carbon source for biohydrogen production in microbial electrolysis cells (MECs). WAS was pretreated by bi-frequency ultrasonic and the highest SCFAs were accumulated at 3rd day. Three groups of tests were conducted in single chamber MECs for H-2 production under different SCOD concentrations. SCOD removals were up to 60% at diluted influent, but reduced to 50% at original concentration. Highest H-2 yield was 1.2 mL H-2/mg COD at 2-fold dilution with 155% energy efficiency. Results showed that >90% of acetate and similar to 90% of propionate were effectively converted to hydrogen, and next were n-butyrate and n-valerate (at dilutions), but <20% of iso-butyrate and iso-valerate were converted. The overall biohydrogen recovery in this study was 120 ml H-2/g VSS/d. This work shows a possibility of cascade utilization of WAS fermentation liquid and H-2 generation in MEC. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.