We have determined with infrared spectroscopic ellipsometry how the nature of the interface between a thin poly(dimethyl siloxane) (PDMS) coating and its substrate affects the rate of PDMS crosslinking reactions. Reactions between vinyl (-CH=CH2) end groups on PDMS and silyl (SiH) groups in a crosslinker (hydrosilylation) and between SiH groups and silanol (SiOH) groups, during the so-called postcure crosslinking stage, have been probed in situ. The overall consumption of SiH follows first-order reaction kinetics. The first-order reaction coefficient (k(1)) for the hydrosilylation crosslinking reaction is the same for coatings on three different substrates: native oxide on silicon (SiO2/Si), polystyrene (PS), and poly(ethylene terephthalate). For the slower posteure reactions, however, the rate of SiH consumption depends on the substrate. In 2.5-mum PDMS coatings on PS, k(1) is about seven times greater than k(1) in the same coating on SiO2/Si. In PDMS coatings on a PDMS substrate, when the effect of the interface is thus minimal, k(1) is 16 times higher than on SiO2/Si. The dependence of k(1) on the type of interface is probably the result of the interfacial segregation and complexation of the Pt catalyst for the postcure reactions. We propose that polar surfaces more strongly attract Pt and form complexes that inhibit the postcure reactions. (C) 2004 Wiley Periodicals, Inc.