In this work, we report dissociation pressures for clathrate hydrates of the methane + n-propyl mercaptan + water and methane + n-butyl mercaptan + water systems in the temperature ranges of 282.3-289.9 K and 279.4-287.4 K, respectively. The experimental data were measured using an isochoric pressure search method. The hydrate dissociation data for both measured systems are compared with some selected literature data for the dissociation conditions of methane clathrate hydrates. It is shown that n-propyl mercaptan has a thermodynamic promotion effect on methane clathrate hydrates while n-butyl mercaptan has a negligible effect.