In this work, we study the effect of the addition of antimony, and chemical sulfurization of CuInSe2 thin films with (NH4)2Sx solution. CuInSe2 thin films were deposited by the molecular beam deposition method and trace amounts of antimony have been added. The antimony, detected by scanning Auger electron microscopy, reacts with the bi- or tri-coordinated In to saturate the dangling bonds on the growing surface as surfactants. During (NH4)2Sx sulfurization, the sulfur atoms enter the thin films through a pathway which is created by the replacement of antimony atoms segregated on the thin film surface. A portion of the selenide film is transformed into an alloy of CuInSe2 and CuInS2 on the grain surface. Significant amounts of sulfur (35%) distributed throughout the thin film of CuInSe2 under an antimony source temperature of 510°C in the film growth was shown from the Auger electron depth profile, and an increase of energy bandgap up to 1.3 eV as determined from optical absorption measurements. The chemical reactions are justified from thermodynamic considerations.