Abstract Fuel grade bioethanol can be produced from a number of lignocellulosic biomass sources. In the present paper the enzymatic hydrolysis of giant reed ( Arundo donax ) at high biomass loadings up to 20% (w/v) was investigated. Two pretreatment technologies were used: one step steam explosion (SE) with and without acid catalyst and the novel pilot process from Chemtex consisting in a two steps pretreatment. The results of the biomass composition after different pretreatment conditions along with the corresponding enzymatic hydrolizability at low solids loading, indicated that the best pretreatments were the acid catalyzed steam explosion (ACSE) at 200 °C and the Chemtex process in which the SE step was carried out at 206 for 4 min. Various process configurations, including biomass and enzymes fed-batch, were investigated to avoid excessive viscosities and increase the cellulose conversion. Novel enzymatic preparations from Novozymes, Cellic™ Ctec1 and Cellic™ Ctec2, were used for the biomass saccharification. Concerning the analysis of the factors that limited the hydrolysis yields at high DM content, some evidences indicated that product inhibition by xylooligomers could have an important role. The highest solids loading investigated in the present paper was 21%. Under these conditions and by using an enzymes loading of 0.28 g Ctec2/g glucan, an overall sugars concentration of 91 g/L (glucose + xylose) can be obtained after 48 h from ACSE at 200 °C.