A dramatic multilayer substrate relaxation is observed for the (root19 x root19)-13CO adlayer phase on a Pt(111) electrode by surface X-ray scattering. Within the (root19 x root19) unit cell, a vertical expansion of 0.28 Angstrom was determined for the Pt atoms under near-to-site CO molecules, whereas only 0.04 Angstrom was found under near-bridge-site CO molecules. The lateral displacements involve small rotations toward more symmetric bonding. Both the expansions and rotations extend into the bulk with a decay length of 1.8 Pt layers. This nonuniform layer expansion, hitherto unseen, appears to be a manifestation of the differential stress induced by CO adsorption at different sites.