The use of Ca(OH)(2) pre-treatment to improve fermentative biohydrogen yields, from wheat straw was investigated. Wheat straw was pre-treated with 7.4% (w/w) Ca(OH)(2) at ambient temperature (20 degrees C) for 2, 5, 8, and 12 days, prior to 35 degrees C fermentation with sewage sludge inoculum. Biohydrogen yields were evaluated during dark fermentation and simultaneous saccharification fermentation (SSF) of total pre-treated straw material and compared to those from separated solid and hydrolysate fractions. Ca(OH)(2) pre-treatment followed by SSF, exhibited a synergetic relationship. On average, 58.78 mL-H-2 g-VS-1 was produced from SSF of pre-treated and filtered solids. This was accompanied by approximately a 10-fold increase in volatile fatty acid production, compared to the untreated control. By omitting pre-treatment hydrolysate liquors from SSF, H-2 production increased on average by 35.8%, per VS of harvested straw. Additional inhibition studies indicated that CaCO3, formed as a result of pre-treatment pH control, could promote homoacetogenesis and reduce biohydrogen yields. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.