Mixed crystals are attractive materials for semiconductor technologies because physical properties such as lattice constants, energy gaps and effective masses can be designed by the control of an alloy composition. This article highlights the optical investigations of mixed crystals. The main themes are focused on infrared absorption and Raman spectroscopies, from which we can estimate a clustering parameter and force constants. Finally, the author introduces the application of Raman spectroscopy to the investigation of InAs_xP_1-x bulk crystals grown by a solution growth method.