Moire phase shift interferometry was used to monitor the concentration distribution of solute in the solution near the growing and melting tetrahydrofuran (THF) clathrate hydrates. During the growth of hydrate from off-stoichiometric solution, the THF concentration near the growth interface changed due to mass transport of guest molecules. During the growth from THF-17H(2)O solution, the THF concentration remained unchanged. This, allows the assumption of formation of stoichiometric THF hydrate, which is structure II. The effects of impurities on the growing and melting hydrates were investigated using the stoichiometric solution with the addition of 3 wt.% sodium chloride. The salt concentration increased with time by the rejection of salt at the growth interface. The freezing temperature of hydrate was considerably reduced by the local increase in salt concentration. The diffusion length of salt was inversely proportional to the growth velocity and determined the characteristic length of the growth pattern. In contrast to this, during the melting process of hydrate, the dilution of solution was observed near the interface. Consequently, the melting process was inhibited due to a rise in the equilibrium temperature of hydrate. (C) 2003 Elsevier B.V. All rights reserved.