Abstract: A heating pack for use with an improved granular chemical composition which produces heat when water is added. The heating pack utilizes an inner and outer bag, the inner bag containing the granular composition and having a closure flap which is partially sealed by means of a barrier envelope which allows water to be added to the inner bag while preventing spillage of the granular composition. The chemical composition contains an improved oxidizing mixture in combination with iron particles which yields a relatively high heat output to weight ratio for the heating pack.