[1 0 0]-Oriented epitaxial and polycrystalline Ba1-xSrxSi2 films with various x values were grown by molecular beam epitaxy (MBE) on Si(1 1 1) and transparent fused silica substrates, respectively. The indirect absorption edge E-edge in Ba1-xSrxSi2 increased with increasing Sr composition, x, and reached approximately 1.40 eV when x = 0.52. However, the E-edge value was almost saturated for higher x values, and the formation of homogeneous Ba1-xSrxSi2 films was difficult for samples in which x > 0.6. Furthermore, phase separation was observed when x = 0.77. Using a thermodynamic model, the average phonon energy in BaSi2 was estimated from the temperature dependence of the optical absorption edge to be approximately 25 meV. (c) 2006 Elsevier B.V. All rights reserved.