From electron spin resonance spectra of a ball-milled cellulose sample, it was established that the mechanical degradation of air-dry cellulose was characterized by the formation of free radicals. It was shown that iodine increased the rate of mechanical degradation, presumably by preventing the recombination of at least some of the radicals formed during grinding. Since no difference in iodine consumption during grinding was observed between air-dry and oven-dry cellulose samples, it was concluded that heterolytic cleavage of cellulose molecules did not occur. The formation of benzyl iodide and cyclohexyl iodide was observed when cellulose was ground with iodine in toluene and cyclohexane, respectively. It is possible that either chain transfer reactions between cellulose radicals and solvent molecules account for the halogenation of the solvent or that the halogenation is due to a hypoiodite structure formed by the reaction of cellulose alkoxy radicals with iodine.