Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm-1 was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on the Si(100) via attachment by one vertex. A complete assignment of the observed vibrartional features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab initio calculations using gradient-corrected density functional methods.