This study aims to investigate the effect of CsI addition on the optical and thermal behavior of Se-S-Ge-Sb-As glasses. For this purpose, the glasses with nominal compositions of Se40S20Ge20Sb5As15 with different percentages of CsI (0 < x < 0.15) were ready by the usual melt-quenching method in quartz ampoule. The glasses were identified using XRD, DSC, UV-Vis-NIR, FTIR, Raman, and IR variable angle spectroscopic ellipsometry analysis. The glass transition temperature of glasses was decreased from 288 °C to 274 °C with increasing the CsI content. The bandgap energy and refractive index of glasses in the existence of CsI were about 1.78–1.72 eV, 2.45–2.40, respectively. Reducing IR transparency, shifting to longer wavelengths of cut-off wavelength, and lowering band gap energy were significant effects of adding CsI to the glass composition.
