Substantial dipoles are formed at interfaces between metals and organic molecules, even in case of relatively weak interactions. We monitor interface dipoles by first-principles calculations of work function changes caused by adsorption of perylene and 3,4,9,10-perylene-tetra-carboxylic-dianhydride monolayers. These changes are the result of two competing effects. Pauli repulsion pushes electrons into the surface, which decreases the work function. If the metal work function is sufficiently low, electrons are donated back from the surface to the molecule. In this regime the work function is effectively determined by pinning of the Fermi level at a molecular energy level.