Anelastic creep curves of 110 symme rical tilt boundaries with mis-orientation angles 60°and 153.4°in aluminium bicrystals has been measured in torsion. It was found that the relaxation strength associated with the boundary becomes zero below a temperature T0 which has a value of about one half of the melting temperature Tm of bulk aluminium. This shows that a perceptible change of the internal structure of the grain boundary has taken place around this temperature.