The effect of thermal treatment on the chemical structure of plasticized poly(vinyl butyral) (PVB) laminated safety glass was investigated in the temperature range 50-200°C using infra-red spectroscopy and viscosity measurements. The infra-red spectra showed a large reduction in both the residual hydroxyl and acetate groups present in PVB, together with the complete disappearance of the cyclic acetal group. Moreover, the results also reveal a continuous decrease in the intrinsic viscosity values with increase in the degradation temperature. The changes in some physical parameters like the contact angle, the degree of swelling and the refractive index as functions of thermal degradation were also investigated. A probable mechanism for the thermal degradation of PVB, based on the experimental results has also been proposed.