Self-heating containers comprise a reaction chamber and a heating chamber. The heating chamber is sized to contain a substance to be heated. The reaction chamber contains reactants which, when contacted, exothermically react. The reaction chamber is divided into a first compartment and a second compartment with a barrier therebetween. The barrier comprises a first barrier portion and a second barrier portion. The first barrier portion is attached to a reaction chamber wall and has an opening sized to allow reactants to flow through from one compartment to the other. The second barrier portion is attached to the first barrier portion to close the opening. The barrier can be opened by moving an actuator into engagement with the second barrier portion to dislodge the second barrier portion from the first barrier portion and thereby open the barrier.