A chemical oxygen or other gas generator which has a container composed of thermally-conductive and gas-impervious material which contains pyrotechnic material, the container being in heat transfer relation with a block of oxygen-producing material for transfer of heat from burning of the pyrotechnic material within the container directly to the block of oxygen-producing material for initiation of decomposition thereof. The container is sealingly attached to the casing so that all of the evolved gases from burning of the pyrotechnic material are segregated from the produced breathable oxygen. The evolved gases are vented from the oxygen generator so as to reduce the ignition chamber pressure and/or allow a greater amount of pyrotechnic material to be burned within the ignition chamber so that a sufficient amount of heat may be generated within the ignition chamber for application to the block of oxygen producing material for initiation of decomposition thereof. This also allows the providing of pyrotechnic material within the space occupied by the oxygen-producing material for initiating decomposition thereof to be desirably eliminated so that a smaller filter capacity is required. The container has a first portion composed of a material which serves as a heat sink for transferring the heat to the block of oxygen producing material of such magnitude and over a period of time to effect initiation of decomposition thereof. A second portion of the container, which is not in direct contact with the pyrotechnic material, is composed of a material for restricting heat transfer to the casing.