X-ray irradiated cloud with density n = 10^5.5 cm^-3 and radiation field F_X = 160 erg cm^-2 s^-1. (left) The abundances are shown for H, H₂, OH, and H₂O in three different cases: formation of OH and H₂O (i) in gas phase only (ii) in gas phase and on dust with an MRN size distribution, and (iii) in gas phase and on dust with a Weingartner & Draine size distribution. (right) Gas and dust temperatures for the same models. The temperature profiles for the gas only models and those where OH and H₂O formation on dust with an MRN distribution are included are the same.