Infrared reflection absorption (IRA) spectra were measured for acrolein adsorbed on an evaporated silver film at 90 K under ultrahigh vacuum conditions. IRA spectra indicated that acrolein adsorbed on the silver substrate exists In a trans form and that, on increasing exposure levels, the adsorbate takes on type-A, -B, -C, and -D structures successively. The adsorption structures are characterized by the C=O stretching (nu(C=O)) and CH2 out-of-plane bending (omega(CH2)) bands; they are observed at 1670 and 978 cm(-1) (type-A). 1693 and 985 cm(-1) (type-B), 1699 and 995 cm(-1) (type-C), and 1691 and 993 cm(-1) (type-D). Type-A and -B interact directly with the substrate, while the type-C and -D form molecular layers on top of the type-A and -B layer. The intensity ratios, omega(CH2)/nu(C=O), observed for type-A, -B -C, and -D are appreciably larger than the ratio observed for acrolein in a polycrystalline state, indicating that all the adsorbates have the molecular plane more or less parallel to the substrate surface. In the CH out-of-plane bending vibration region, type-C and -D give the CH out-of-plane bending bands of the vinyl group and formyl group near 1018 and 1005 cm(-1), respectively, in addition to the omega(CH2) band, while type-A and -B give only the prominent band due to omega(CH2). This trend was observed also for acrylic acid lying flat on a silver substrate (Fujii, S.; et al. Surf. Sci. 1994, 306, 381). Thus, the selective enhancement of the omega(CH2) band seems to be a general feature for a planar molecule containing a vinyl group and interacting directly with the silver surface. Ab initio molecular orbital method at the RHF/4-31G* level was applied to calculate the normal vibrational frequencies of a free acrolein as well as a silver ion-acrolein complex; the results reproduced frequency differences between IRA bands of type-A and the corresponding bands of acrolein in a gas state. proving that type-A forms a coordination bond between the oxygen atom of the C=O group and a positively charged site of the substrate.