Apparatus for monitoring items passing through an entry/exit opening in a delimited space, comprising: radio-frequency transmission means (2) adapted to communicate with a plurality of transponders associated with said items to detect the presence of the transponders in proximity of the opening, each transponder being adapted to transmit a specific identifier code, at least a pair of optoelectronic sensors (3, 4) arranged in series at said opening, each sensor (3, 4) being adapted to transmit a first signal identifying a condition of clear sensor, in which said sensor (3, 4) detects the opening as being clear, and a second signal identifying a condition of locked-on, i.e. engaged sensor, in which said sensor (3, 4) detects an item passing through said opening. Control means (5) are provided to continuously store and/or update the time-based detection events of the transponders and the time-based variation events of the signal transmitted by the sensors (3, 4), said control means (5) being adapted to compare said time-based signal-variation events to determine an opening passing-through time interval (T), and to compare the time-based detection events of the transponders with the passing-through time interval (T) to determine the transponders that have passed through the opening.