Brewers' spent grain (BSG), the main low-value solid residue, is the major by-product of the brewing industry, representing around 85 % of the total by-product generated. This raw material is renewable, at low cost, largely available throughout the year and produced in large quantities. BSG was hydrothermally treated by instantaneous controlled pressure drop process (DIC) at five pressure levels (or five temperatures), with as objective to observe the effect of the processing pressure on enzymatic hydrolysis of BSG. Enzymatic hydrolysis was performed on the pretreated solids by celluclast (1.5L). The results showed that the hydrolysis yield of BSG treated at pressure levels of 2, 3, 4, 5 and 7 bar for a processing time of 15 min was strongly improved compared to hydrolysis yield of unpretreated BSG. The best hydrolysis yield was obtained, up to 100%, when the BSG was pretreated at more intensive conditions (5 and 7 bar for 15 min). The morphology of treated BSG samples was studied with scanning electronic microscopy (SEM). Untreated BSG exhibited rigid and highly ordered fibrils whereas the structure of pretreated BSG at 7 bar seems to have an important disruption.