A mathematical model for pretreatment and hydrolysis of lignocellulosic biomasses in twin-screw extruders is presented. The model allows for calculation of the concentrations of extracted hemicellulose and sugars (xylose and glucose) at different operating conditions (pressure, temperature, and NaOH and H2SO4 concentrations, as well as screw rotational speed). Parameter sensitivity analyses on the effects of process variables on product concentrations are reported. Regarding the pretreatment case, the effects of NaOH concentration, liquid/solid ratio, and screw rotational speed on the extracted hemicellulose are studied. In the case of hydrolysis, the effects of NaOH concentration, screw rotational speed, and barrel temperature on the final concentrations of glucose and xylose are addressed. Reasonably good agreement between experimental data from different pretreatment/hydrolysis systems and laboratories and the calculations obtained with our model is observed.