The reactions of 1,3-dienes with the Si(001) surface have been investigated using scanning tunneling microscopy (STM) and Fourier transform infrared spectroscopy (FTIR), and the relative efficiencies of [2 + 2] and [4 + 2] reactions have been determined. STM and FTIR studies show that the 2,3-dimethyl-1,3-butadiene molecule has two bonding configurations; 80% of the molecules bond via a [4 + 2] reaction involving both alkene groups with the remaining 20% bonding via a [2 + 2] reaction involving only one alkene group. The molecule 1,3-cyclohaxadiene shows three separate bonding configurations in the STM, and the FTIR shows at least four separate peaks in the alkene stretching region. The [4 + 2] product is found to comprise 55% of the surface species, the [2 + 2] product 35%, and an unknown product 10%. The surface temperature is found to have little affect on the product distribution. The formation of multiple products and the lack of temperature effects indicate that the product distribution is controlled primarily by the kinetics of the adsorption process, not by the thermodynamics. Thus, although [4 + 2] reactions are predicted to be more stable, [2 + 2] reactions occur nearly as frequently.