Dissociation processes of methane hydrate under waterflow conditions were investigated by a combination of experimental observations and numerical simulations using computational fluid dynamics (CFD). In Part I of this study, the dissociation process induced by water flow at pressures above the three-phase [hydrate (H)-liquid water (L-w)-vapor (V)] boundary in an isothermal x-P phase diagram is discussed. Dissociation experiments were carried out with a methane hydrate ball (diameter congruent to 10 mm) suspended in a flow cell, and the overall dissociation rate of methane hydrate without bubble formation was measured under various conditions of pressure, temperature, and volumetric flow rate of water. A linear phenomenological rate equation in the form of the product of the dissociation rate constant k(bl) and the molar Gibbs free energy difference Delta G, between the hydrate phase and the ambient aqueous phase, was derived by considering the Gibbs free energy difference as the driving force for the dissociation. The molar Gibbs free energy difference was expressed by the logarithm of the ratio of the concentration of methane dissolved in water at the hydrate surface to the solubility of methane in the aqueous solution in equilibrium with the hydrate. The dissociation rate constant k(bl) was determined from the experimental results of the overall dissociation rate combined with the numerical simulation results of the concentration profile of methane by the CFD method. The obtained dissociation rate constant was found to be independent of the ambient water flow rate, indicating that the rate constant is intrinsic for the hydrate dissociation within the conditions examined in this study. The rate constant was independent of the pressure, whereas the temperature dependency was described by an Arrhenius-type equation with the apparent activation energy of 98.3 kJ/mol. (c) 2006 American Institute of Chemical Engineers.