A proper discussion on the possibility and feasibility of technological applications for gas hydrates requires knowledge of the phase behaviour and its relation to the gas hydrate structure and its occupation. This paper presents experimental data on gas hydrate phase equilibria for the system water + propane and for various systems of the kind water + propane + additive. The additives considered are tetrahydropyran, cyclobutanone and cyclohexane, which are assumed to occupy the large cavity of structure H (sII) hydrate, and methylcyclohexane that is a typical structure H (sH) hydrate former. All additives have in common that they are very poorly soluble in water and, therefore, an additional liquid phase is present in these systems. The pressure for the equilibrium hydrate-liquid water-vapour (H-L-w-V) in the system water + propane is reduced upon addition of each of these components. Simultaneously, the hydrate equilibrium hydrate-liquid water-liquid propane (H-L-w-L-C3H8) is shifted to lower temperatures. These observations can be explained in terms of mutual miscibility of propane and the additive component. However, it cannot be excluded that propane molecules are exchanged by additive molecules in occupying the large cavity of sII.