The phosphitylation of cellulose with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane [P(II)], is proposed as a means to determine its reactivity via an evaluation of its accessible hydroxyl groups. A variety of cellulose samples were subjected to this phosphitylation reaction, and the consumption of phosphitylation reagent was followed by quantitative P-31 NMR spectroscopy. This consumption was found to be directly proportional to the amount of reactive hydroxyl groups on the cellulosic material. To further evaluate the quantitative reliability of this methodology, cellulose samples were subjected to a series of mechanical beating treatments, and the changes in the amount of accessible OH groups were evaluated. In addition, cellulose samples were equilibrated to various moisture contents, and their accessible OH groups were determined using the developed methodology. Both variables examined were found to affect the amount of reactive OH groups present on the samples with variations in the moisture content having a greater effect. For example, up to 6.5 mmol g(-1), of accessible OH groups were found to be created within the highly refined samples at the highest moisture content.