An attempt has been made to form continuously either a structure-I or a structure-H hydrate using methane as the common guest substance and methylcyclohexane as the second guest for the structure-H hydrate. The experimental technique we tested was to spray water into a high-pressure chamber charged with methane gas. In the experiments to form the structure-I hydrate, water droplets sprayed from a single nozzle at the top of the chamber coalesced into a water pool underlying the methane gas phase. In the experiments to form the structure-H hydrate, the water droplets fell onto a methylcyclohexane layer superposed on a pool of water then merged with the pool. Water was continuously drained from the bottom of the chamber and circulated back through the spray nozzle. During this circulation, the water passed through a heat exchanger outside the chamber to release the heat generated by hydrate formation. The pressure in the chamber was maintained at a prescribed level (typically 2.5-3.7 MPa) by supplying methane gas through a port at the top of the chamber. The rate of methane flowing into the chamber was continuously measured to determine the instantaneous rate of hydrate formation in the chamber. The results of the measurements indicate that comparable methane-storing rate may be obtained for structure-H and structure-I hydrates, but with the structure-H hydrate formed at lower pressures. The process of hydrate formation was observed visually through a pair of glass windows in the chamber wall, alternatively using a digital video camera and a microscope connected to a highspeed video-recording system. This paper discusses the mechanisms of hydrate formation revealed by these observations and by the measurements of the rate of hydrate formation.