The invention relates to a mask for administering oxygen or other breathable gas to the nose and mouth of a patient. The invention relates to improvements in gas delivery masks, and specifically an improved diffuser structure which is particularly suitable for use in an open mask system, in which the mask body includes substantial openings that allow the user to freely converse, drink, and perform other functions. The improvements also relate to a rebound chamber within the diffuser body spaced apart from the gas conduit outlet, the rebound surface being configured to rebound and reverse the direction of flow of at least a substantial portion of the gas stream exiting the outlet from the forward direction to a rearward direction towards the user's face.