The diffusion of aluminium and ytterbium in Yb2O3–Al2O3–SiO2 glasses has been investigated between 1700 and 2000°C with concentrations of up to 3 mol% Al2O3 and 0·8 mol% Yb2O3. The diffusion coefficient of aluminium is independent of the ytterbium concentration and can be described by an Arrhenius function with a constant preexponential factor, D0=101·48 cm2s−1, and a concentration dependent activation energy, E=(497−47cAl2O3 0·41) kJ mol−1 which decreases with increasing molar concentration of Al2O3. For concentrations greater than 0·5 mol% Al2O3, the diffusion coefficient of ytterbium is virtually the same as that for aluminium. At aluminium concentrations lower than 0·5 mol% Al2O3 and ytterbium concentrations lower than 0·1 mol% Yb2O3, ytterbium diffuses independently of aluminium with a modified diffusion coefficient, with a pre-exponential factor D0=100·48 cm2s−1, and an activation energy E=424 kJ mol−1.