We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sl) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate hydrate was investigated by time-of-flight neutron powder diffraction at temperatures ranging from 230 to 263 K and pressures up to 5000 psi (34.5 MPa). These samples were prepared from deuterated ice crystals and transformed to hydrate by pressurizing the system with argon gas. Complete transformation from D2O ice to sII Ar hydrate was observed as the sample temperature was slowly increased through the D2O ice melting point. The transformation of sII argon hydrate to A hydrate was achieved by removing excess Ar gas and exposing the hydrate to liquid CO2 by pressurizing the Ar hydrate With CO2. Results suggest the sI hydrate formed from CO2 exchange in argon sII hydrate is a mixed Ar/CO2 hydrate. The proposed exchange mechanism is consistent with clathrate hydrate being an equilibrium system in which guest molecules are exchanging between encapsulated molecules in the solid hydrate and free molecules in the surrounding gas or liquid phase.