We studied the optical properties of platinum octaethyl porphyrin (PtOEP) emitting layer in single-layer organic light emitting diode (OLED) device and in thin film grown oil quartz plate using a phase modulated spectroscopic ellipsometry. Spectra of optical constants (refractive index if and extinction coefficient k) were obtained for PtOEP in the visible-ultraviolet spectral range. Difference was observed in the it and k spectra between the thin film evaporated on quartz plate and the encapsulated OLED. Additionally, difference was found between the layer thickness estimated by quartz oscillator and thickness estimated from the ellipsometry measurement. Discussion is given on contamination by oxygen and moisture in the unsealed thin film. (c) 2005 Elsevier B.V All rights reserved.