The coarsening of Al₃Sc precipitates in an Al–1 mass%Mg–0.27 mass%Sc alloy aged at 673 to 748 K has been studied by measuring both the precipitate size by transmission electron microscopy (TEM) and the Sc concentration in the Al matrix by electrical resistivity measurements. The growth and coarsening of Al₃Sc precipitates occur concurrently prior to the beginning of coarsening. The Al₃Sc precipitates grow more slowly at the coarsening stage than at the mixed stage of growth and coarsening. The Al/Al₃Sc interface energy γ and the diffusivity D of Sc in Al have been independently derived from data obtained by TEM and electrical resistivity measurements using a coarsening model developed by Kuehmann and Voorhees for ternary systems. The estimates of γ and D are in agreement with those determined by the Lifshitz-Slyozov-Wagner theory from data on coarsening of Al₃Sc precipitates in an Al–0.28 mass%Sc alloy. The experimentally obtained value of γ is insensitive to the change in coherency between the Al₃Sc precipitate and Al matrix. Whether the precipitates are coherent or semi-coherent with the matrix, the obtained value of γ is then 0.23 J/m².