Pseudo-lignin is formed during dilute acid pretreatment (DAP), particularly under high-severity conditions, and has been shown to significantly inhibit enzymatic hydrolysis of cellulose. To suppress its formation, DAP was modified by performing it under O₂ or N₂; adding surfactant (Tween-80) to the reaction mixture; or using a water–dimethyl sulfoxide (DMSO) mixture as reaction medium. Pseudo-lignin analysis showed that only the addition of DMSO to DAP reaction medium can effectively suppress pseudo-lignin formation. This was attributed to DMSO preferentially solvating and stabilizing 5-hydroxymethyl furfural (HMF) which is the key intermediate to form pseudo-lignin, thereby reducing the overall yield of pseudo-lignin. Furthermore, the addition of DMSO was shown not to reduce pseudo-lignin molecular weight or change any of its structural features significantly. Therefore, pseudo-lignin generated from aqueous DMSO DAP had similar inhibition properties as compared to that acquired from routine DAP at equal mass dosages. This study is the first demonstration that the amount of pseudo-lignin formed during DAP can be reduced, which contributes to further optimization of DAP technology.