A method is presented for the determination of the coefficient of thermal expansion of solids, particularly in the low‐temperature region. The method is based on the measurement of the resonant frequency of one or more modes of a microwave cavity made of the material for which the coefficient of expansion is required. A change in length of 5 parts in 106can be detected. This accuracy is primarily determined by theQof the resonant cavity. Measurements of the thermal expansion of copper and barium titanate are presented over the temperature range of −200 to +100°C and the former is compared with observations made by other techniques.