This work focuses on the effective exploitation of agricultural residues within the biorefinery concept. Lignocellulosic biomass has several uses but its intertwined nature hinders complete exploitation. Thus, to achieve total utilization of the key lignocellulosic components (cellulose, hemicelluloses and lignin) a crucial step is the pretreatment. Several pretreatment methods have been proposed over the years each one having certain advantages and disadvantages. Organosolv pretreatment process of wheat and barley straw, with various organic solvents, is proposed and evaluated herein. The two approaches evaluated to simulate the fractionation process are the Severity factor and the Response surface methodology. The results of the organosolv fractionation process of wheat and barley straw towards enzymatic digestibility are presented. The influence of the most important process parameters is investigated in an attempt to achieve process optimization. Furthermore, the adsorption capacity of the untreated and pretreated wheat and barley straw, are extensively studied using several kinetic and isotherm equations and models. Finally, general conclusions and aspects for further research are presented.