Based on the mixing energies in Figure we would expect Bi2(Se1-xSx)3 to form stable solid solutions in both the Pnma and R3 ̅ m phases. Since we calculate the absolute free energies of each of the alloy compositions, we can also use our model to predict which of the two are most stable for a given composition and formation temperature (Figure ). We find that at most temperatures and compositions the Pnma phase is the most energetically stable, with the R3 ̅ m phase favored up to around x = 1/2 (25-50 % S) below ~550 K, above which it is the most stable phase only very close to the SnSe endpoint (x = 0). Experimentally, Bi2(Se1-xSx)3 is reported to form a Pnma solid solution with compositions from Bi2S3 (x = 1) to the Bi40Se32S28 (x = 0.47), Pnma phases with composition Bi2Se2S and Bi2SeS2 (x = 0.33/0.67), and an R3 ̅ m solid solution from Bi2Se3 to Bi40Se56S4 (𝑥 = 0.07). This is remarkably consistent with our predictions for synthesis temperatures above ~600 K.