The expansion coefficients of specimens under tensile stress and after the stress has been removed have been measured for three steels (0.20, 0.40, 0.80% carbon) and for two types of Invar. If the applied stress is below the elastic limit the change in expansion coefficient &agr;, with stress &sgr; is shown to be given by(∂&agr;/∂&sgr;)T=−1/E2(∂E/∂T)&sgr;, whereEis Young's modulus andTthe temperature; if the stress is removed the expansivity returns to its original value. When the applied stress exceeds the elastic limit, there is a discontinuous change in expansion coefficient, and the expansivity on unloading differs from the original value. The lattice distortion associated with the onset of even a slight plastic strain (2×10−6) appears sufficient to change the expansion coefficient both under load and after the load is removed.