Two different types of polycrystalline films, films with low (Fe) and films with high (Co50Fe50) saturation magnetostriction λs, were sandwiched between Cu to study the thickness dependence of λs in Cu/X/Cu systems. We utilised a new method (ROTMOKE) for a high accurate Hk determination. The values for X, were determined by analysing the change of Hk during application of compressive and tensile stress by cylindrical bending of the substrate joint with the film. For both systems, a remarkable contribution of the interface to the λs, was found. The sign of the interface contribution is opposed to the sign of bulk λs, leading to a zero transition of λs., For Fe this transition occurs at a thickness of 5 nm whereas for the CoFe film with a strong λs, value of the bulk of around 80 · 10−6 a transition was extrapolated at a thickness around 1 nm.