Abstract Thin films of hydroxypropyl methylcellulose (HPMC) are prepared by solution casting technique and exposed to UV/ozone for different exposure times. The effect of exposure on the optical properties such as: near infrared (NIR), transmittance spectra in the spectral region 250–2500 nm of the films are studied. The changes in the optical parameters including: The CIE tristimulus values, color parameters, absorption coefficient, absorption edge, band tail width, optical band gap, extinction coefficient and color strength as well as refractive index, dispersion parameters and optical dielectric constant are determined as a function of UV/ozone exposure times. The results indicate that, the NIR spectra showed variations in the intensity, area, band width, and the absorbance values of some bands. These variations mean that there are changes in the molecular configuration as well as the bond vibration and structure of HPMC as the exposure time increases. It is also noticed from the data that the variations in the values of band tail and optical band gap with increase the exposure time may be due to HPMC-induced structural change in the system. In addition, it is recognized that exposure with UV/ozone plays a role in microstructure and macrostructure change occurring in the polymer matrix.