This article analyzes the effects of dilute sulfuric acid and steam explosion pretreatments on the fractionation and kinetics of the dilute acid hydrolysis of the cellulosic fraction in different biomass substrates. A total of nine biomasses were analyzed : three hardwoods, two softwoods, and four types of herbaceous material. Sigmacell, a purified microcrystalline cellulosic substrate, was also studied as a reference. It is concluded that steam explosion is a more efficient pretreatment for the fractionation of hardwood and softwood substrates with a low or moderate level of cellulose degradation. Under the experimental conditions however, the steam explosion caused a dramatic degradation of the hemicellulose sugars. The dilute acid prehydrolysis produced much less hemicellulose sugar degradation and revealed a more advantageous method, producing the subsequent acid hydrolysis of the cellulose because of its positive effect (with the exception of sweet sorghum bagasse) on the hydrolysis rate for all the substrates studied, although this seems to be primarily linked to the minimization of the neutralizing capacity of the biomasses as a consequence of the prehydrolysis step. In contrast, the steam explosion pretreatments always produce a decrease of the subsequent cellulose acid hydrolysis rates, which can be related to the significant increases of the crystallinity index values shown by the steam-exploded cellulose.