The adsorption of vinylacetic. acid (VAA) on the Si(001)2x 1 surface has been investigated using low-energy electron diffraction, X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. VAA adsorbs on the Si(00 1)2 x I surface at room temperature, not through the reaction of the vinyl group (CH2=CH-) but through the dissociative adsorption of RCOOH (R = CH2 = CHCH2) into RCOO(ad) as a unidentate adsorbate and H(ad) without breaking the Si dimers. Curve-fitting of the Si 2p core level spectra taken after VAA exposures shows the development of two new surface components, So and S-H, shifted by +0.93 and +0.40 eV, respectively, from the bulk component. The binding energy shifts indicate that the So and SH components are due to the Si-OOC-R and Si-H species formed by the dissociative adsorption of VAA. The adsorption behavior was confirmed by the analysis of the C Is and 0 Is core level spectra. The C Is core level spectrum for the VAA-saturated surface clearly shows three components contributed by carbon atoms in the O=C-O, -C-C-, and -C=C-groups, while the 0 Is core level spectrum consists of two components due to oxygen atoms in the Si-O- and -C=O groups. There is no evidence for the bonding between the Si dimer and the -C=C- bond of VAA through the [2+2] cycloaddition reaction, which is expected in the adsorption of unsaturated hydrocarbon compounds on the Si(00 1)2 x 1 surface, nor for the hetero-[2+2] reaction of the -C=O group with the Si dimer. This selectivity in the reacting group is discussed on the basis of the differences in reaction barriers between the Si dimers and the functional groups of the VAA molecule.