The dependence of the metal-carbon vibrational frequency (nu(M-C)) upon electrode potential (E) for saturated CO adlayers on palladium, platinum, rhodium, and iridium film electrodes is examined in comparison with that for the well-studied intramolecular (C-O) vibration (nu(CO)) by means of surface-enhanced Raman spectroscopy (SERS) in order to evaluate the likely roles of chemical bonding versus the electrostatic field in the electrochemical Stark effect. In each case, the d nu(M-C)/dE values are negative, from ca. -10 to -20. cm(-1) V-1, contrasting the positive d nu(CO)/dE values, ca, 30 to 60 cm(-1) V(-)1, observed for adsorbed CO on these Pt group metals. The findings are compared with the predictions of theoretical treatments which account variously for the roles of the interfacial electrostatic field (i.e., the classical vibrational Stark effect) and potential-dependent chemical bonding (i.e., metal-adsorbate orbital overlap). It is necessary to invoke that the latter factor is exerting a major role in the surface-adsorbate interactions in order to account for the observed nu(M-C)-E dependences.