The adsorption of formic acid (HCOOH), acrylic acid (CH2= CHCO2H), propiolic acid (HC=CCO2H), and the corresponding alcohols on the Si(111)(7×7) surface have been investigated by NEXAFS. In each case, well-defined dipole transitions to σ* and π* molecular orbital s were observed above the C and 0 K-edges and used to probe the orientation and chemistry of these molecules on this silicon surface. 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 of these molecules do not react initially with the silicon surface. Upon heating, however, the C-C double and triple bonds which are held in proximity to the surface by the carboxylic acid or alcohol group, are lost either by polymerization on the surface or reaction with the silicon substrate. These results illustrate the capabilities of NEXAFS to investigate molecular orientations on surfaces and the electronic structure of polyatomic adsorbates.