The interstitials cause the bending of a bimetallic strip if they diffuse perpendicularly to its plane. The formula for the bending vs. time is presented, respecting the thicknesses of both components of the bimetal, their coefficients of diffusion and Young's modula. The bending curves, computed for several values of parameters involved in the process, are compared to the experimental results in the case of diffusion of hydrogen through the double-layer Ni-Cu.