The hydrate-former system composed of methane-propane-water was studied through HP-mu DSC and a PVT cell. New equilibrium data were obtained. Results reveal evidence of the simultaneous formation of structures I and II, although only sII is expected at such conditions at equilibrium. Most of sI is present as a metastable structure, but a small fraction of it seems to provide a more stable system, mainly at excess methane conditions. The effect of heating rate, mixing, multiple cycles and initial gas phase composition on the formation of both structures was evaluated. These factors affect the formation kinetics, the type and the proportion of each hydrate structure and hence the thermodynamic profile during dissociation. Raman spectroscopy was applied for studying ex situ single and double hydrates, suggesting a higher fraction of sII than sI for all gas phase compositions studied, and higher occupancy of large cavities by methane molecules as methane content increases. (C) 2019 Elsevier Ltd. All rights reserved.