A study has been undertaken of the gas-grain chemistry of protostellar disks which are sufficiently cool that in the outer regions, where the gas density is less than ~1013cm-3 and the ionization rate highest, a bimolecular chemistry resembling that of dark clouds can occur. Since the gas-grain collision rate is so high, outgassing mantle molecules effectively determine the gas phase composition at any position in the disk. In contrast to previous work, a detailed gas phase chemistry is considered along with the accretion and desorption of mantle species which is controlled locally by the dust temperature.