A new infrared ratio, α1372 cm.−1/α2900 cm.−1, is proposed for measuring crystallinity in cellulosic materials. The advantage of this ratio over others which have been used is that it can be applied to both celluloses I and II and, therefore, to samples containing a mixed lattice. Two series of samples, encompassing a wide range of crystallinity, were prepared from highly crystalline celluloses I and II. The infrared ratios of these samples were compared with crystallinity values from x-ray diffractograms and density measurements, and with accessibility data from moisture sorption. It was shown that the new infrared ratio ranks samples of both lattice types, as well as partly mercerized cottons, in the same order as do x-ray, density, and moisture sorption data. The correlation of the new infrared ratio with accessibility, derived from moisture regain, is better than with crystallinities from either x-ray or density measurements. Reasons for this are suggested.