Binary H-2-THF clathrate hydrate formation kinetics were investigated with a pressure decay method at temperatures from 266.7 to 275.1 K, at initial pressures from 3.6 to 8.4 MPa, and at stoichiometric THF hydrate concentrations for particle sizes between 212 and 1,400 mu m. Formation rate increased for smaller particle sizes, higher pressures and lower-temperatures. A hydrogen delocalization model and a proposed hydrogen hydrate phase diffusion (HHPD) model were used to analyze the formation mechanisms. The HHPD model assumes that the H-2-THF hydrate phase is formed due to hydrogen adsorption onto the particle surface that is followed by subsequent diffusion of hydrogen into the clathrate hydrate. The HHPD model could express the kinetics quantitatively at the experimental conditions studied. Values of the hydrogen diffusion coefficient in the clathrate hydrate estimated from the bulk data and the phase thickness in the HHPD model agreed well with the literature. (C) 2008 American Institute of Chemical Engineers AIChE J, 54: 3007-3016, 2008