Adsorption of acetylene on the Si(100)-(2 x 1) surface was studied at room temperature by scanning tunneling microscopy (STM) and theoretical modeling. Acetylene molecules adsorb nondissociatively in three distinctive bonding configurations, not one, as reported previously. In Configuration 1, C2H2 molecules adsorb in di-sigma form over a single silicon dimer with a relative population of 0.33 at low coverage. Configuration II has an initial formation probability of 0.51 and involves adsorption across the ends of two adjacent dimers in the same row. In Configuration III (r-bridge) C2H2 forms four bonds as it symmetrically bridges two neighboring dimers. Relative populations depend strongly on coverage. Above a coverage of similar to0.13 an acceleration in the rate of formation of Configuration III (r-bridge) is observed.