With the use of X-ray powder diffraction data and the Rietveld method, we verified that both microcrystalline cellulose PH-101 and PH-102 presented differences in the crystallinity degree. The results revealed these samples are semicrystalline in nature and via Rietveld refinements, it was possible, adding a known amount of an internal standard of corundum to the samples, to perform quantitative phase analyses, which allowed us to determine the amorphous amount of the studied samples: 63.7(11) wt % in microcrystalline cellulose PH-101 and 51.0(11) wt % in microcrystalline cellulose PH-102. An important contribution of this work refers to the attempt of using this simple method, permitting to evaluate the degree of crystallinity of microcrystalline cellulose. © 2014 Wiley Periodicals, Inc. and the American Pharmacists Association.