This paper reports a set of Sellmeier equations for GaS and GaSe that provide excellent reproduction of the phase-matching conditions for second- to sixth-harmonic generation of CO₂ laser radiation at 10.5910 μm in GaS(0.4)Se(0.6) at 20°C. In addition, this set of Sellmeier equations is found to reproduce well the phase-matching angles for second-harmonic generation of a Ti:Al₂O₃ laser-pumped BaB₂O₄ optical parametric generator at 2.14-2.9 μm and CO₂ laser radiation at 9.2(9.2007)-10.7(10.6746) μm measured by Kang et al. [Appl. Phys. B 108, 545 (2012)] in GaS(0.09)Se(0.91) and GaS(0.41)Se(0.59).