In a method or field device for determining a position of an open/closed-valve positioned by a pneumatic actuating drive and movable to a completely opened open position and a closed position depending on operating requirements, during normal operation a pneumatic working chamber of the actuating drive is pneumatically influenced with an i/p converter such that the open/closed-valve moves to one of the open/closed positions and, in case of a predetermined operating condition, the i/p converter allows the working chamber to vent so that the open/closed valve automatically moves to the other one of the open/closed positions. At least one of pressure prevailing in the working chamber and pneumatic control pressure output at the i/p converter is captured continuously over time, thus obtaining a pressure change over time on the basis of which a physical operational variable of the open/closed-valve is determined.