An equimolar mixture of hydrogen and iodine atoms adsorbed on the Si(100) surface is generated by adsorption of iodoethane and annealing to 570 K (which causes ethyl groups to undergo hydrogen elimination and ethylene to desorb). Multiple internal reflection-Fourier transform infrared spectroscopy (MIR-FTIR) shows that the adsorbed hydrogen is distributed among several configurations. After annealing to 700 K, the spectrum shows only a single configuration, which is assigned to surface dimers that are occupied by one hydrogen and one iodine atom. Density functional theory calculations indicate that these mixed-occupation dimers are not energetically favored over other doubly occupied configurations (two H atoms or two I atoms). This evidence indicates that, in contrast to other ordering phenomena in surface adlayers, the ordering in this case is a kinetic effect and is not driven by thermodynamics.