Abstract: A balloon intended to be swallowed and to occupy a given volume of the stomach in the presence of other balloons of the same type in order to permit a weight reduction of an individual is made up of an envelope containing two substances capable of reacting chemically with each other and with water in order to form a gas permitting the inflation of the balloon. The two substances are isolated by a coating melting under the effect of human body temperature. Other applications may be foreseen for a balloon capable of inflating automatically under the effect of a predetermined temperature.