The effects are explored of time-dependent element diffusion on the mode-trapping properties of DA white dwarf models with various thicknesses of the hydrogen envelope. Our predictions are compared with the standard assumption of diffusive equilibrium in the trace-element approximation. We find that element diffusion markedly weakens the presence of mode trapping originating in the outer layers of the models, even for the case of thin hydrogen envelopes.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plata