A thermal diffusion process is discussed, that should be expected in a
ny system in which the difference in the local susceptibilities chi(T)
- chi(S) is inhomogeneous, where the superscripts indicate isothermal
and adiabatic processes. That constitutes an universal secondary resp
onse of disordered systems which must be considered in conjunction wit
h the primary ''mechanical'' response. The response function due to th
ermal diffusion behaves asymptotically as e(-(t/tau)1/2) for short tim
es and as t(-3/2) for long times, but can be interpolated by e(-(t/T)0
.60) for a wide range of times around T. The thermal diffusion process
can be particularly relevant in the response of magnetic glasses, ele
ctric dipole glasses, and in multidomain ferroelectric/ferromagnetic s
ystems with more than two domain orientations.