An experimental determination of the thermo-optic effect of titanium dioxide is described in which the test element is a thin film deposited onto the end face of an optical fibre. The film acts as an etalon. The reflectivity of titanium dioxide films was measured as a function of temperature experimentally over the range 20°C to 500°C, and modelled theoretically to yield a value of the thermo-optic coefficient of -1.49 × 10-4 K-1. The use of the data in the design of a thin-film optical fibre temperature sensor is described. © 1997 Elsevier Science Ltd.