Abstract: A heat-sensitive alarm trigger is used to set off a fire alarm system. The alarm circuit is triggered either when a resistance across two sensor leads, between which the alarm trigger is connected, falls below the rated trigger resistance or when an emf of a certain strength is produced between two electrodes of the trigger. The heat-sensitive alarm trigger comprises a laminate structure which includes an optionally perforated first electrode layer and a second electrode layer. A hydrated material layer is disposed between the two electrode layers. When the hydrated material is heated above a given alarm trigger temperature, moisture is given off and the barrier layer becomes sufficiently conductive so as to trigger the alarm. This is either caused by lowering the resistance of the hydrated material to below the rated trigger resistance or by producing a sufficient emf between the electrodes which have different electrode potentials.