A method for monitoring transportation of bills ensures reliable control of the transportation processes. Each bill (BN) is routed through a light barrier that has a light-sensitive sensor (S) to detect at least one edge of the bill (BN). To suppress detection of windows (W) and to detect the rear edge (HK) of the bill (BN), the signal (S0) produced by the sensor (S) is debounced. The debounce time (dT) is set based on the length of the bill (BN), to a first period (dT1) that corresponds to the expected length of the bill (BN). The debounce time (dT) is set to the first period (dT1) as soon as the signal (S0) produced by the sensor (S) indicates the first appearance of an edge, particularly the front edge (VK), of the bill (BN). The debounce time can be set to a second shorter, period (dT2) after the first period (dT1) has elapsed.