We study the adsorption geometry of 3,4,9,10-perylene tetracarboxylic dianhydride on Ag111 and Cu111 using x-ray standing waves. The element-specific analysis shows that the carbon core of the molecule adsorbs in a planar configuration, whereas the oxygen atoms experience a nontrivial and substrate dependent distortion. On copper silver the carbon ring resides 2.66 Å 2.86 Å above the substrate. In contrast to the conformation on Ag111, where the carboxylic oxygen atoms are bent towards the surface, we find that on Cu111 all oxygen atoms are above the carbon plane at 2.73 Å and 2.89 Å, respectively.