Abstract: The monitoring device serves to monitor the formation of bookblocks at depositing stations for collating machines and to release a control impulse at faulty formation of a bookblock. The object of the invention is to accomplish the formation of bookblocks on collating machines, provided with suction drums, and to reduce expenses of adjusting the machine for processing of varying paper thicknesses. The object is to construct a device, in such a manner that the precalculated position of a contact lever may be simply adjusted upon a base plate corresponding to any paper thickness and that the smallest deviations of position of the contact lever in one or another direction may be deleted while ensuring regularity of signal.According to the invention, this object is accomplished by fastening at least one spring mounted stop for setting the precalculated position of the contact lever upon a base plate which is movably disposed relative to the machine frame.