Abstract: An apparatus (14) for developing an exposed proof (16) provided with a visible light absorbing composition such as color particles or pigments includes a transport means (22-28, 36) for transporting the proof (16) along a treatment liquid dispenser unit (30, 32) and a take-off device (44, 38, 40) for removing detached composition from the proof (16). A tempering device (48) is provided for keeping the treatment liquid, which in the dispenser unit (30, 32) is applied onto the proof (16), at the optimum temperature for the developing process. The tempering device (48) comprises at least one Peltier element (52) which has at least one of its two tempering faces thermally connected with treatment liquid. The Peltier element (52) is adapted to receive a reversible supply voltage whose amount and polarity can be controlled. Depending on the polarity of the supply voltage, the Peltier element (52) operates either as a heating element or a cooling element.