A compressed-air operated dispenser for continuously dispensing a substance enclosed in a container, and in particular a viscous or paste-like substance, comprising an outlet passage extending from container to an outlet; a valve which normally closes the passage; a compressed-air operated piston arranged to exert pressure on a substance in the passage and to open the valve; and a trigger means for initiating the supply of compressed air for the purpose of operating said piston.In accordance with the invention there is provided a second valve arranged to move the piston to a starting position in which a pressure-actuating chamber is closed on one side of the piston with the exception of a passage through which compressed air can be forced into the chamber to cause the piston to move to a first terminal position. In this position the substance is subjected to pressure and the valve is opened and the piston ceases to act on the second valve. An air-return passage is opened to cause the second valve to be moved to a second position, in which an exhaust hole for compressed air in said chamber is placed in communication with atmosphere and the piston returns to a second terminal position in which it causes the second valve to return to its starting position.