Control of the reactivity of natural polymers – such as semicrystalline cellulose – through polymorphic transitions is a potent, yet underexplored tool in modern polymer science. Here, the degradation behavior of three artificial cellulose polymorphs (cellulose II, IIII, and IIIII) in the presence of hydrogen chloride vapor is explored. While the ultimate results of hydrolyses correspond to those found for aqueous HCl, the kinetic scission models exhibit a unique trend for each polymorph, unlike those reported for aqueous acid or enzymatic hydrolyses. In addition to the polymorphic distinctions, these atypical trends are attributed to the nonequilibrium in the hydrolysis set up and the irregular adsorption of HCl molecules to the substrate surfaces. The results point to a new way of approaching the reactivity of natural polymers where polymorphism is regarded as one of the parameters for the kinetics and outcome of chemical reactions.