This study discusses the S-doping effects on WO3 structure and optical constants. The SxWO3 system (x = 0, 0.05, 0.1 and 0.2) was prepared by easy sol-gel method and heat-treated at 500 degrees C for 8 h. The X-ray diffraction pattern revealed the formation of a single orthorhombic cell for x = 0, 0.1 and 0.2 compositions, while for x = 0.05 a monoclinic unit cell was identified. The variation of the cell parameter and the FTIR analysis (441 cm(-1), W=S terminal) confirmed the doping process. The refractive index (n), the extinction coefficient (k), electrical (sigma(e)) and optical conductivity (sigma(opt)), dissipation factor (tan delta) and dielectric relaxation time (tau) were investigated. The study demonstrated that the S-doping improved some optical constants, particularly light transport properties. Moreover, high S-doping generated defects, which led to decreased electrical conductivity and charge carriers lifetime.