The dissociation kinetics of methane and methane-ethane hydrates was investigated under a variety of experimental conditions. Hydrates of pure methane or methane-ethane mixtures were prepared. The composition and structure of methane-ethane mixed hydrates were identified using Raman spectroscopy and gas chromatographic analysis of the hydrate phase. With these hydrate powders, pellet-shaped samples that mimic a naturally occurring hydrate in ocean sediment were prepared. The dissociation rates of gas hydrates were measured in pure water and a viscous fluid mixed to imitate drilling mud fluids under several isothermal and isobaric conditions. Gas bubbles generated by dissociation affected the dissociation rate, possibly because gas bubbles near the active surface resisted heat flux during dissociation. For methane-ethane mixed hydrates, the calculated time profile agrees well with the experimental results when the composition of the vapor phase is identical with that of the hydrate phase. It indicates that the free gas composition around the dissociation surface is determined by the kinetics of dissociation and not by thermodynamic equilibrium. The dissociation rates of gas hydrates in viscous fluids were essentially proportional to the concentration of fluid.