Any pretreat process used on lignocellulosic substrates, such as steam pretreatment, although opening up and enhancing access to the cellulose, will typically generate inhibitory compounds (ex. soluble mono/oligomeric sugars, phenolics, furans, and extractives) that will limit or restrict the efficiency of cellulose hydrolysis. Although water washing can be used to remove these soluble inhibitors, it consumes large amounts of water and energy, adds processing steps and reduces the final sugar concentration after enzymatic hydrolysis. To better develop more efficient inhibition mitigation strategies, it would be beneficial if we could better understand the inhibitory mechanism of these soluble compounds on the enzyme components present in cellulase cocktails. Most previous studies which have tried to assess the effects of inhibitors on cellulase enzymes have used “synthetic mixtures of inhibitors”, and have focused on the “traditional” cellulase preparations such as Celluclast. The work that will be presented assessed the major inhibitory compounds derived from a range of “real-life” steam pretreated lignocellulosic biomass treated at different pretreatment severities and how the various fractionated components affect the rate and extent of enzymatic hydrolysis when using more recent cellulase mixtures. Possible inhibitory mechanisms such as, reversible/irreversible inhibition, enzyme adsorption/precipitation of the major enzyme activities (exo/endo-glucanase, β-glucosidase, xylanase activities, etc.), influence of sugars, phenolics, etc., were assessed and possible solutions to dealing with the different inhibitors will be described.