Radio-frequency (RF) tissue fusion is a novel method of tissue approximation that can seal tissue without the need for sutures or staples, based on the combined effects of heat and pressure on the apposed tissue surfaces. RF delivery must be controlled and optimized to obtain a reproducible, reliable seal. We use real-time optical measurements to improve understanding of the tissue modifications induced by RF fusion. The main macroscopic transformations are thermal denaturation and dehydration. Light propagation in tissue is a function of both and therefore should provide interesting insight into the dynamic of occurring phenomena. Quantification by continuous wave technique has proven challenging. We proposed an algorithm based on the measurement of the absolute transmittance of the tissue, making use of the modified Beer-Lambert law. The experimental method and the data algorithm are demonstrated by RF fusion of porcine small bowel. The proposed optical measurement modality is well adapted to modern surgical instrumentation used for minimally invasive procedures.