We have developed the traveling heater method (THM) using a low melting point solvent to grow alloys of GaInSb at constant composition with low defect density. The melting point is reduced by using excess antimony or indium as a solvent. By growing the crystal at low temperature it is possible to avoid the high concentration of gallium antisite defects responsible for the dominant native acceptor concentration. With reduced acceptor concentration, a significant increase in the optical transmission from 2 to 20 mum was observed in crystals grown at low temperature. A series of crystal growth runs has demonstrated the use of off-stoichiometry melts as a means to control the electrical and optical properties of GaInSb alloy crystals.