A detailed investigation of the Hall mobility has been carried out on a series of [inverted lazy s]2‐&mgr;‐thickn‐type {001}Si/{011¯2}Al2O3films. A specially designed Hall bridge pattern has been used to obtain independent measurements of mobility as a function of current direction in the plane of the film. The data show an anisotropy in the mobility of approximately 9%, with a maximum in mobility occuring along the ⟨100⟩ Si direction that is parallel to the ⟨21¯1¯0⟩ Al2O3direction in the plane of the substrate. This behavior is found to be a consequence, through the piezoresistance effect, of the anisotropic thermal contraction of Al2O3on cooling from the deposition temperature, which leads to an anisotropic thermally induced stress in the Si.