Abstract: A self-heating or self-cooling container for food or drink comprises inner and outer vessels defining a reaction cell between their mutually facing walls is provided. The reaction cell is divided into first and second chambers by a rupturable membrane. A liquid chemical reagent is filled into the second chamber which is at ambient pressure, and a solid chemical reagent is placed in the first chamber which is at sub-atmospheric pressure. The sub-atmospheric pressure in the first chamber causes a rapid and thorough mixing of the chemical reagents, and shaking the container after the initial rupture of the membrane assists that rapid mixing. Preferably the second chamber is elastically deformable to increase its internal volume, for example being formed with corrugated walls. Alternatively the reaction cell may be provided with pressure relief valve means.