Carbohydrate-derived pseudo-lignin formed during dilute acid pretreatment can slow down the conversion of cellulose into sugars. Even at moderate levels of severity, hemicellulose-derived pseudo-lignin can hinder the breakdown of cellulose by enzymes. When a mixture of xylan and cellulose is pretreated, the presence of xylan can reduce the conversion of cellulose significantly more than expected due to cellulase inhibition. This study shows that the insoluble degradation products from hemicellulose can play a key role in reducing the efficiency of converting biomass into sugars.