LiGaSe2 and LiInSe2 are promising nonlinear crystals for conversion of laser radiation to the mid-IR spectral range which are transparent down to the visible and UV. We successfully grew a new mixed crystal as a solid solution in the system LiGaSe2 - LiInSe2, with a composition of LiGa0.5In0.5Se2 which has the same orthorhombic structure (mm 2) as the parent compounds (LiGaSe2 and LiInSe2). The new crystal is more technological with regard to the growth process in comparison with LiGaSe2 and LiInSe2 since its homogeneity range is broader in the phase diagram. We established that about 10% of the Li ions are found in octahedral position with coordination number of 3. The band-gap of LiGa0.5In0.5Se2 is estimated to be 2.94 eV at room temperature. The transparency at the 0-level extends from 0.47 to 13 μm. The dispersion of the principal refractive indices was measured and Sellmeier equations were constructed. The fundamental wavelength range for the SHG process extends from 1.75 to 11.8 μm. The nonlinear coefficients of LiGa0.5In0.5Se2 have values between those of LiGaSe2 and LiInSe2.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.