We present a periodic density functional theory study of the energetic profile of the fluorination of dichloromethane over the (0 0 0 1) surface of alpha-alumina. This is a model system for the industrially important fluorination process used in synthesis of fluorohydrocarbon replacements of the environmentally detrimental chlorofluorcarbons. The results indicate that HF is readily, and strongly, chemisorbed to the alumina surface producing a surface fluorine ion and a hydroxyl group. Using this model of the fluorinated surface we calculate the adsorption energy of dichloromethane and estimate the barrier to its reaction to CH2ClF through an S(N)2 scheme with the surface fluorine acting as a nucleophile. From the energy profile generated we propose a kinetic scheme and estimate the expected experimentally observed barrier for the overall process. To test the calculation results we have also carried out experimental studies of dichloromethane fluorination over alpha-alumina and show that the calculated and measured activation energies are in good agreement.