The change in the optical properties of tissue during thermal treatment can be potentially used to monitor procedures like Radiofrequency Ablation (RFA). We present key features in the optical absorption and scattering of tissue during the RFA procedure and during post-ablation cooling down to room temperature. We have used time-resolved diffuse optical spectroscopy for the measurement of the optical properties of tissue for the wavelengths from 650 to 1100 nm. Ex vivo experiments were conducted using a clinical RFA system on bovine liver tissue. Measurements were performed for two temperatures (70°C and 105°C). The following features were observed in the optical properties. First, there was a decrease in optical absorption and an increase in scattering during the treatment. With overtreatment, the absorption increased for initial part of the spectrum (until 910 nm) and scattering decreased in comparison to normal treatment. Secondly, a redshift of the hemoglobin peak and blue shift around water peak was observed in the optical absorption. Finally, a new peak around 840 nm and a valley around 920 nm appeared with heating. When the tissue was allowed to cool down, most of the changes in the absorption around the water peak partially reversed including the blue shift and the valley around 920 nm. Additionally scattering decreased with cooling. Results show key features in the optical properties of tissue during RFA, the effect of overtreatment and post-treatment cooling in ex vivo tissue. Insights from this study will help in advancing optical methods in monitoring thermal treatment.