The reaction of a silicon adlayer deposited on Pt(111) by chemical vapor deposition (CVD) using silane (SiH4) is described. Data from Auger electron spectroscopy (AES) reveal that Si readily diffuses into the Pt substrate and sequentially forms at least two unique intermetallic Pt-Si surface structural phases with (root 7 x root 7)R19.1 degrees and (root 19 x root 19)R23.4 degrees real space unit cells as characterized by low-energy electron diffraction (LEED). The chemisorption properties of each of the ordered overlayers were studied using CO as a molecular probe. Reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) studies indicated that CO was mostly limited to chemisorption at "atop" Pt sites in the root 7 phase with a significant reduction of the heat of adsorption with respect to the Pt(111) surface. The root 19 phase also showed a significant modification of the chemisorption properties although it is not as pronounced as that seen for the root 7 structure. The relevance of these studies to intermetallic thin film growth is discussed.