The goal of this work is to understand and further develop an in-situ preparation method for producing high quality polymer thin films. In this two-step vacuum process, a monomer film is deposited onto a cooled substrate, and then the polymerization reaction is initiated; thus the polymerization is confined to the substrate. Compared to other preparation techniques, this process has the major advantages of allowing the fabrication of films from insoluble polymers and providing better control of film quality. This investigation focused on the molecular orientation and packing of the monomer, as well as polymerization kinetics and film quality characterization. In-situ adsorption and polymerization studies were done using Reflection-Absorption Infrared Spectroscopy (RAIRS) and Temperature-Programmed Desorption (TPD). Time resolved FTIR spectra were taken to study the factors controlling the rate of polymerization. GPC, ellipsometry and AFM provided information about molecular weight, film thickness, and morphology characterization of the films. The model systems investigated were styrene and methyl methacrylate (MMA) on platinum. For the styrene/Pt model system, kinetic studies reveal that the polymerization reaction is preferred in amorphous monomer layers. Ellipsometry measurements show that, in the range of 25-200nm, the polystyrene film thickness is a linear function of the monomer dosage. AFM data suggest that the resulting film morphology is influenced by the platinum substrate. Polarized RAIRS spectra of MMA show that, up to 0.2 micron, only p-polarized component of the beam contributes to the spectrum, indicating that RAIRS can be used to determine the molecular orientation of films with sub-micron thickness.