The chain conformations of cyclo-olefin polymer (COP) and polystyrene (PS) in less than 200-nm thick films on silicon wafers were investigated on the basis of the refractive index measured by multi-angle spectroscopic ellipsometry (MASE), and density measured by X-ray reflectometry (XRR). For both COP and PS, the density measured by XRR increases by decreasing the film thickness to below 50 nm. Densification may be caused by close packing of unentangled polymer chains in ultrathin films spincast from dilute solutions with polymer concentrations less than the overlap concentration (C*). For COP films, the refractive indices at incident angles of 45 degrees and 70 degrees measured by MASE agree well with those calculated by the Lorentz-Lorenz equation, indicating that densification of COP ultrathin films enhances their refractive indices. For PS films thinner than 50 urn, although the refractive index at an incident angle of 45 degrees agrees with a calculation based on the Lorentz-Lorenz equation, one at 70 degrees significantly deviates downward. A comparison of them with the results of quantum chemical calculation (QCC) suggested a plane-arrangement of benzene rings in PS ultrathin films, which was likely brought about by stacking of benzene rings and attractive interaction between pi-electrons in the benzene rings and the substrate surface. (C) 2012 Elsevier Ltd. All rights reserved.