The optical response of granular Tl‐Ba‐Ca‐Cu‐O films has been used to investigate thermal properties of the films. An analysis of the response using a heat transfer model yields a thermal diffusivity D=10−3 cm2/s at 150 K which rises to 6×10−3 cm2/s at a temperature of 30 K and allows for an estimation of the boundary resistance Rbd≊10−3 K cm2/W between film and substrate. The dependence of the response time on film thickness obtained from the heat transfer model is compared with published data indicating that in many experiments the observed response is mainly bolometric in origin.