The methane uptake and conversion rate to structure H (sH) hydrates was measured and compared to crystallization kinetics models. Three large molecule guest substances (LMGS) were used as sH hydrate formers: neohexane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The initial crystallization occurred quickly at the LMGS liquid-ice interface until similar to 20-30% of ice was converted into hydrate (hydrate growth stage I). Slower hydrate crystal growth was observed after a hydrate film covered the ice surface at a rate of 3-400 nm(2)/h (hydrate growth stage H). The TBME system showed the fastest kinetics at the beginning of the reaction followed by NH and MCH system. However the trend changed when the temperature was increased ("reaction" stage III). Surprisingly, the conversion rate achieved with the TBME system upon melting the ice was the smallest. This was attributed to the strong interaction of TBME with water molecules that increased the energy barrier for water molecules to form hydrate cages. The conversion rates were well correlated with the Avrami equation and the shrinking core model. Finally, NH was found to be the best LMGS in this study to obtain full conversion within a short reaction time and achieving high methane gas storage in the hydrate. (C) 2007 American Institute of Chemical Engineers.