Highly adhering plasma-polymerized diphenyl (PPDP) thin films were grown by a dry and flawless process in glow discharge. The structural analyses of as-deposited and heat treated PPDP samples (unaged and aged) were performed by infrared (IR) spectroscopy. The PPDP thin films were transparent and light yellow in colour. Optical properties were evaluated from ultraviolet-visible (W-Vis) spectroscopic measurements of as-deposited, heat treated and aged (as-deposited and heat treated) PPDP thin films. From the optical absorption data band gaps, allowed direct and indirect transition energy gaps were determined. The results are discussed in the light of possible structural modification in PPDP thin films on heat treatment and aging.