Abstract: In a method and a device for controlling the blanket cylinder washing processes of a rotary offset printing machine with several printing units (d1) each assigned a washer unit (12-27) and several reel changers (33) each connected to a reel changer control unit (34), spoilage is reduced by simple, operator-friendly, cost-effective and universal means when washer units (12-27) are connected to the outputs of a washer unit control (40), a trigger signal (A) is applied at one output of the washer unit control (40) and at the output of a logical AND-function of the reel changer signal (R) and of a counting signal (Z) of a counting device (38) for the printed copies, and a memory is provided for the current data set with distances from the lead reel changer (33) to the first-triggered washer units (22, 23, 24, 25) assigned to the web coming from the lead reel changer, and the respective distances from all washer units (12-27) to the folder, the web speed and the web widths, so that the wash spoilage of all webs oc