Abstract: Deep eutectic solvents (DESs) are emerging as sustainable alternatives to conventional solvents due to their environmental compatibility and molecular tunability. In this work, we investigate the temperature- and structure-dependent refractive index behavior of a binary DES composed of choline chloride and acetic acid (1:2 molar ratio) modified with three structural isomers of butanol—1-butanol, 2-butanol, and 3-butanol. Refractive index (nᴰ) measurements were performed from 293.15 to 323.15 K using an Abbe-type digital refractometer.