The mobility of surface electrons localized over a superfluid helium film covering a solid substrate is calculated by taking into account the essential scattering processes of electrons by helium atoms, by ripplons and by the helium-substrate interface roughness. The last scattering process dominates for thin helium films with thickness with d approx.< 10~(-6) cm. A new scattering Hamiltonian, recently derived by the method of image forces, is employed. The calculated results for the mobility are compared with experimental data by fitting the two characteristic sizes of the Gaussian correlation model for surface defects. The agreement is rather good in the case of a glass substrate.
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