Methyl mercaptan, CH3SH, is an industrial waste as well as the reactive product of several H2 and H2S induced catalytic hydrogenation processes of COS and CS2. Its coupling into value added products is of great importance in monetizing sour natural gas. In the present work, the full theoretical cycle of catalytic CH3SH coupling to form ethene was investigated by means of density functional theory (DFT) using chabazite as a model catalyst with emphasis on the first C-C bond formation. Calculated thermodynamics were compared with those of analogous and well established CH3OH processes to identify the similarities and differences in the reactive pathways. With few exceptions, CH3SH catalytic transformations are of higher free energy when compared to those of CH3OH. The trimethylsulfonium ion, TMS, isostructural with that of the trimethyloxonium ion, TMO, is shown to be a key reactive intermediate and a thermodynamically stable species leading to ethene formation.