An inflatable flotation device having a reaction chamber with reactants that generate gas when mixed, said reactants being separated by a barrier assembly; an activating mechanism adapted to remove or puncture the barrier assembly such that said reactants mix; and an inflatable compartment fluidly connected to the reaction chamber by means of a pressure sensitive passageway, adapted to open only when the pressure of the gas in said reaction chamber exceeds a predetermined threshold. This ensures that most of the reactants are used up to generate gas before the gas enters the inflatable chamber, thus preventing scattering of the reactants. Some implementations have a reaction chamber without a barrier assembly, comprising reactants that generate gas when mixed with water and an activating mechanism adapted to expose the reactants to water. A variety of manual or automatic mechanisms may be employed to activate the reaction and inflate the device.