Cellulose and hemicelluloses are the most abundant structural polysaccharide in nature. Their optimal valorization in lignocellulose-based biorefineries requires a good knowledge of the contents available in various biomasses, and of their molecular composition. The composition and more specifically the monosaccharidic content (cellulosic glucan, xylan, arabinan, mannan, galactan and hemicellulosic glucan) were analyzed in several lignocellulosic grass crops: switchgrass (Panicum virgatum L.), tall fescue (Festuca arundinacea Schreb.), cocksfoot-alfalfa association (Dactylis glomerata L.-Medicago sativa L.). Cellulose and hemicelluloses contents were in the range 20.8-33.3 and 14.5-32.6 g (100gDM)-1, respectively, with lignin, total soluble sugars, proteins, and ash contents around 2.9-7.8, 2.0-21.7, 5.7-17.2 and 4.3-16.3 g (100gDM)-1, respectively. The main hemicellulosic components were, by order of decreasing importance, xylan 8.3-23.2 g (100gDM)-1, arabinan 2.12-4.10 g (100gDM)-1, hemicellulosic glucan 1.36-3.42 g (100gDM)-1, galactan 0.63-2.31 g (100gDM)-1 and mannan 0.27-1.08 g (100gDM)-1. Plant species and harvest cycle has the most significant influence on the structural polysaccharide composition, while the influences of cultivar and nitrogen fertilization were only rarely significant.