The effects on mechanical properties of heterogeneous acetylation and methylation of isotropic cellulose films have been studied. The effects on breaking load, extensibility, yield point, elastic recovery, and initial Young's modulus in the air-dry state were very small. It is concluded that these properties are not markedly influenced in cellulose derivatives by variation of the nature of the intermolecular cohesive forces in the way that might have been anticipated from the analogy of a simple molecular network. It is suggested that the lack of effect of such treatments, and also the cause of the low elastic recovery in cellulose and its derivatives, may be connected with molecular stiffness, dipole distribution, and supracrystalline structure. Cellulose and its derivatives are contrasted with nylon in this respect. The effects of methylation or acetylation on mechanical properties in water are much more extensive and are attributed to a solubilizing effect which, in the case of progressive acetylation, is ultimately opposed by the formation of acetate-acetate intermolecular bonds that are relatively stable to water.