  We study the adsorption geometry of
3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) and Cu(111)
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 non-trivial and substrate dependent distortion. On copper
(silver) the carbon rings resides 2.66 A (2.86 A) above the substrate. In
contrast to the conformation on Ag(111), where the carboxylic oxygen atoms are
bent towards the surface, we find that on Cu(111) all oxygen atoms are above
the carbon plane at 2.73 A and 2.89 A, respectively.
