H/D and C-12/C-13 vapor pressure isotope effects in methyl fluoride (CH3F) were studied at the MPW1PW91/6-31++G(d) level of theory. The CH3F molecule and small CH3F clusters were used as models of vapor and liquid methyl fluoride, respectively, and their reduced partition function ratios were calculated at their optimized structures. The equilibrium constants of the isotope exchange reactions between the two phases were evaluated as 1.0263 and 0.9991 for the H/D and C-12/C-13 isotope exchanges at 150 K, respectively, which reproduced reasonably well the experimental results. The effect of CH center dot center dot center dot F interaction between neighboring molecules on the vpies was discussed. (c) 2007 Elsevier B.V. All rights reserved.