Wheat straw is gaining importance as a feedstock for the production of biofuels and high value-added bioproducts. Several pretreatments recover the fermentable fraction involving the use of water or aqueous solutions. Therefore, hydration properties of wheat straw fibers play an important role in improving pretreatment performance. In this study, the water retention capacity (WRC) and swelling of wheat straw fibers were studied using water, propylene glycol (PPG) and an effluent from a H-2-producing reactor as the hydration media with three particle sizes (3.35, 2.00 and 0.212 mm). The effects of swelling were analyzed by optical and confocal laser scanning microscopy (CLSM). The highest WRC was reached with the effluent medium (9.84 +/- 0.87 g g(-1) in 4 h), followed by PPG (8.52 +/- 0.18 g g(-1) in 1 h) and water (8.74 +/- 0.76 g g(-1) in 10 h). The effluent hydration treatment had a synergic effect between the enzymes present and the water. The particle size had a significant effect on the WRC (P < 0.01), the highest values were reached with 3.35 mm fibers. The CLSM images showed that finer fibers were subjected to a shaving effect due to the grinding affecting its capacity to absorb the hydration medium. The microscopic analysis showed the increase in the width of the epidermal cells after the hydration and a more undulating cell wall likely due to the hydration of the amorphous regions in the cellulose microfibrils. The sugar release was determined, achieving the highest glucose content with the effluent hydration treatment. (C) 2014 Elsevier Ltd. All rights reserved.