Both untransformed poplar and genetically modified “zip-lignin” poplar, in which additional ester bonds were introduced into the lignin backbone, were subjected to mild alkaline and copper-catalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment. Our hypothesis was that the lignin in zip-lignin poplar would be removed more easily than lignin in untransformed poplar during this alkaline pretreatment, resulting in higher sugar yields following enzymatic hydrolysis. We observed improved glucose and xylose hydrolysis yields for zip-lignin poplar compared to untransformed poplar following both alkaline-only pretreatment (56% glucose yield for untransformed poplar compared to 67% for zip-lignin poplar) and Cu-AHP pretreatment (77% glucose yield for untransformed poplar compared to 85% for zip-lignin poplar). Compositional analysis, glycome profiling, and microscopy all supported the notion that the ester linkages increase delignification and improve sugar yields. Essentially no differences were noted in the molecular weight distributions of solubilized lignins between the zip-lignin poplar and the control line. Significantly, when zip-lignin poplar was utilized as the feedstock, hydrogen peroxide, catalyst, and enzyme loadings could all be substantially reduced while maintaining high sugar yields.