The adsorption of formic acid (HCO2H), propanoic acid (CH3CH2CO2H), acrylic acid (CH2=CHCO2H), propiolic acid (HC≡CCO2H), and the corresponding alcohols on the Si(111)(7×7) surface have been investigated by NEXAFS. Studies of condensed layers of these molecules show that, in general, the NEXAFS spectrum of a polyfunctional molecule is simply the superposition of NEXAFS features expected of the individual functional groups in the molecule. An exception to this rule was observed for those molecules with conjugated π orbitals. Monolayer coverages of these molecules on silicon, bond strongly to the silicon surface via the carboxylic acid or alcohol group. In contrast, the C-C double and triple bonds do not react initially with the silicon surface. Upon heating, however, the C-C double and triple bonds attached to the silicon surface by the oxygen functional groups become reactive on silicon. These results illustrate the capabilities of NEXAFS to investigate the electronic structure and chemical reactivity of polyatomic adsorbates. © 1987 Elsevier Science Publishers B.V.