Finally, analysis of the 𝜅 latt of the alloys using approximate models suggests that the reduction in the thermal conductivity of R3 ̅ m Bi2(Se1-xSx)3 is likely to be driven predominantly by mass disorder, whereas strain-field fluctuations due to inhomogeneous chemical bonding play a significant role in the reduced 𝜅 latt of Pnma Bi2(Se1-xSx)3 and R3 ̅ m Bi2(Se1-xTex)3. Nonetheless, we find that using an isotope-scattering model to predict the effect of mass disorder allows for useful qualitative predictions. A more detailed analysis of the thermal transport in Bi2SeTe2 indicates that disordered occupation of the chalcogen atoms away from the preferred sites, favored by configurational entropy, is likely to be an important factor in the reduced 𝜅 latt of this compound compared to Bi2Te3 observed in experiments.