The relationships between solvent polarity in binary mixed solvents, as measured by the solvatochromic ET values, and various thermodynamic and kinetic data have been investigated with a view to judging the presence of preferential solvation effects. The free energies of transfer, ΔGt, of simple inorganic ions from water to water–propanol, water–acetone, water–THF mixtures, and the ΔGt values of whole electrolytes from water to water–methanol, water–propanol, water–acetone and water–THF mixtures do not correlate in a linear fashion with ET values. On the other hand, ΔGt data for the solute 1-naphthoic acid transferred from water to water–ethanol mixtures vary linearly with ET values and it is suggested that this is due to preferential solvation by ethanol. The activities of the organic co-solvent in water–organic solvent mixtures do not correlate linearly with ET. The rates (as log k) of a range of reactions in mixed solvents correlate linearly with ET and it is suggested that preferential solvation plays a significant role in influencing the solvent dependence of reaction rate. Anomalous results are observed for the correlation of rate data in the water–methanol system and this is discussed.