Dissociative adsorption of methyl iodide at 300 K on Si(100) was followed by multiple-internal reflection Fourier transform infrared spectroscopy. The absorption spectrum shows only two peaks in the C-H stretching region (2890 and 2955 cm-1) which are assigned to the symmetric and antisymmetric C-H stretching modes of methyl bonded to silicon. The intensities of the v(CH3) modes are very weak and exhibit a strongly nonlinear dependence on coverage. The results of isotope labeling studies suggest that methyl groups at minority sites dominate the spectral intensity. Thermal annealing studies indicate that the kinetics of methyl decomposition at temperatures above 600 K is affected by co-adsorbed halogen and hydrogen atoms.