Reactive magnetron sputter epitaxy was used to deposit thin solid films of Sc1-xAlxN (0≤x≤1) onto MgO(111) substrates with ScN(111) seed layers. Stoichiometric films were deposited from elemental Sc and Al targets at substrate temperatures of 600 °C. The films were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection analysis, x-ray diffraction, and transmission electron microscopy. Results show that rocksalt structure (c)-Sc1-xAlxN solid solutions with AlN molar fractions up to ∼60% can be synthesized. For higher AlN contents, the system phase separates into c- and wurtzite structure (w)-Sc1-xAlxN domains. The w-domains are present in three different orientations relative to the seed layer, namely, Sc1-xAlxN(0001)||ScN(111) with Sc1-xAlxN||ScN, Sc1-xAlxN(1011)||ScN(111) with Sc1-xAlxN||ScN, and Sc1-xAlxN(1011)∥ScN(113). The re- sults are compared to first-principles density functional theory calculations for the mixing enthalpies of c-, w-, and zinc blende Sc0.50Al0.50N solid solutions, yielding metastability with respect to phase separation for all temperatures below the melting points of AlN and ScN.