As a newly-developed method for pretreating lignocellulosic biomass, phosphoric acid plus hydrogen peroxide (PHP) was employed as a pretreatment solvent to fractionate wheat straw. The structural properties of the derived lignin were elucidated in particular. Results indicated that 100.0 g of wheat straw (dry basis) yielded 39.7 g of cellulose-rich fraction, 4.7 g of oligosaccharides, and 4.5 g of lignin at mild conditions of 50 degrees C within 1.0 h. The resultant cellulose-rich fraction was highly accessible to hydrolytic enzymes with 88-96% cellulose-glucose conversion in 24 h, suggesting a great potential for producing biofuels. The derived lignin was characterized as having high purity (<= 1.0% residual carbohydrates) and relatively low molecular weight (M-w < 1436 g mol(-1)) and being abundant in carboxylic acid functional groups. According to the P-31, C-13, and 2D heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy results on the derived lignin, the degree of condensation was quite limited during PHP pretreatment, the oxidants, produced as HO+ or HO center dot in pretreatment, were substantially responsible for the extensive ring-opening on the aromatic substructure. The obtained results offered the technical feasibility of fractionating lignocellulosic biomass using PHP and a better understanding of the delignification mechanisms for PHP pretreatment.