The effects of mantle desorption due to cosmic rays on the chemistry of interstellar clouds are modelled. Two desorption mechanisms are considered - direct cosmic ray heating of dust grains and cosmic ray induced photodesorption. Their effects at densities typical of a molecular cloud and a dense core are investigated. It is found that both mechanisms have a significant effect on the gas-phase molecular abundances. The results for the molecular cloud case are compared with observations of TMC-1 and found to be in reasonable agreement.