The thermally induced vibrations of rectangular plates and beams under some typical heat applications are studied. The basic parameter of the problem, B, is found to depend on the natural frequency of the structure and on a characteristic thermal time. Curves are presented of the variation with B of the ratio of the deflections calculated including and neglecting the effect of inertia. The role of inertia is found to be important for rapidly applied heat inputs and for thin plates. An approximate formula for its rapid estimation is also presented.