The thermal behaviour of some antiallergic drugs belonging to different groups of antihistaminics, such as phenyltoloxamine, chlorpromazine, clemizole and meclozine was examined. Thermogravimetry, derivative thermogravimetry and differential thermal analysis curves were used for the elucidation of the mechanisms of thermal decomposition. The melting temperatures of the compounds and their thermal stabilities were determined. The stability was found to decrease in the order: clemizole > chlorpromazine > meclozine > phenyltoloxamine.