The stability of an asymmetrically substituted copper phthalocyanine (AsyCuPc) Langmuir-Blodgett (LB) film gas-sensitive element which showed good selective sensitivity to NH3 in air was inspected by an aging test on the device. The reasons leading to the change in response characteristic with time are discussed. It is indicated that the change in gas-sensing response of the AsyCuPc LB film probably arises from the variation in film structure due to the influence of some external factor such as environmental temperature and vibration.