We used spectroscopic ellipsometry to analyze the refractive index and the absorption coefficient of thin buffer layers of InP grown by metalorganic vapor phase epitaxy on Si substrates. We found a pronounced influence on the crystallographic properties of the subsequently grown InP main layer. The drastically increased optical absorption of the buffer layers is possibly caused by a high density in misfit dislocations or twins originating from the difference in lattice constants of InP and Si.