Addition of water to the levo- and meso-isomers of 2,3-butanediol increases the specific gravity to a maximum at 50 to 60% diol, beyond which it is gradually reduced. Contraction on mixing is strongly influenced by temperature and is slightly greater with the levo- than with the meso-isomer. The coefficient of thermal expansion for the levo-isomer is higher than that of the meso-form. Water exerts a pronounced influence on the specific rotation value of the levo-isomer, reducing it from − 13° (anhydrous) to approximately − 9° for solutions containing 40 to 70% water; on further dilution, the specific rotation value rises. With increase in temperature, the optical rotatory power of the anhydrous levo-isomer is reduced, whereas that of 30 to 90% aqueous solutions is increased. Refractive index data are provided for aqueous meso-2,3-butanediol solutions at 25 °C. and for aqueous levo-2,3-butanediol solutions at 20°, 25°, 30°, and 35 °C. From several lines of evidence it is concluded that the hydration tendencies of the two isomers are approximately equivalent.