The optical properties of thin films of perylene-monoimide (PMI) and perylene-diimide (PDI), which are well known organic semiconductors, were determined using the spectroscopic measurements. For this purpose, optical measurements such as the absorption and reflection of films were performed in near ultraviolet, visible and near infrared spectral regions. Optical band gap of PMI and PDI were determined by means of absorption measurement. In addition, from the absorption and reflection spectra, the dispersion of several optical parameters of materials such as the absorption coefficient and the refractive index were calculated. Single oscillator dispersion analysis was performed based on the Sellmeier equation and some relevant dispersion parameters obtained. © 2014 Elsevier B.V.