•GaAsSb layers were grown using Ga-As-Sb and Ga-As-Sb-Bi melts by LPE.•Bi is found to control the Sb/As ratio in GaAsSb epilayers.•Layers grown using Bi melt show improved crystalline quality. Bi is used for the first time to control the Sb content in GaAsSb epitaxial layers, grown by liquid phase epitaxy (LPE) with high Sb in the layers (>10 at%). This is evidenced by a steady decrease of the Sb incorporation in the layers, obtained from high resolution X-ray diffraction (HRXRD) measurements, with increased Bi addition to the growth melt. X-ray photoelectron spectroscopy (XPS) is used to study the different surface bonding states of the constituting elements of the epilayer, and the results suggest that Bi is not incorporated in the layer but reside over the surface of the material, in metallic or in oxide states. A band gap reduction of 220 meV is measured in GaAsSb layer containing 11 at% Sb, by low temperature photoluminescence (PL) spectroscopy. Addition of Bi to the growth melt resulted in a blue shift of the band gap upto 40 meV. From low temperature PL and temperature dependent PL, it is observed that the luminescence intensity increases and the full width at half maximum (FWHM) decreases for the layers, grown from Ga-As-Sb-Bi melts compared to layers grown from the a melt without Bi, suggesting improved surface and crystalline quality of the epitaxial layers.