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We investigate the effect of alloying with scandium on microstructure, high-temperature phase stability, electron transport, and mechanical properties of the Al 2 CoCrFeNi, Al 0.5 CoCrCuFeNi, and AlCoCrCu 0.5 FeNi high-entropy alloys. Out of the three model alloys, Al 2 CoCrFeNi adopts a disordered CsCl structure type. Both of the six-component alloys contain a mixture of body-centered cubic ( bcc ) and face centered cubic ( fcc ) phases. The comparison between in situ high-temperature powder diffraction data and ex situ data from heat-treated samples highlights the presence of a reversible bcc to fcc transition. The precipitation of a MgZn 2 -type intermetallic phase along grain boundaries following scandium addition affects all systems differently, but especially enhances the properties of Al 2 CoCrFeNi. It causes grain refinement; hardness and electrical conductivity increases (up to 20% and 14% respectively) and affects the CsCl-type → fcc equilibrium by moving the transformation to sensibly higher temperatures. The maximum dimensionless thermoelectric figure of merit ( ZT ) of 0.014 is reached for Al 2 CoCrFeNi alloyed with 0.3 wt.% Sc at 650 °C.