In this work, by quantifying the respective contributions of amorphous and crystalline cellulose to the X-ray diffraction spectra of cellulose with intermediate degrees of crystallinity, a new method to obtain consistent crystallinity index values was developed. Multivariate statistical analysis was applied to X-ray diffraction spectra obtained from phosphoric acid pretreated cellulose samples of various intermediate (but undetermined) crystallinity indices to reduce the dimensionality of the spectra data. The crystallinity indices obtained were found to be linearly related to the enzymatic hydrolysis rates. Validation of the method was done by predicting the degree of crystallinity of samples containing various ratios of microcrystalline cellulose and amorphous cellulose, both of known crystallinity indices. Dimensionality reduction of the spectra was also used to predict the enzymatic hydrolysis rates of various cellulose samples from X-ray data. The method developed in this work could be generalized to accurately assess the degree of crystallinity for a wide range of varieties of cellulose.