Abstract: According to the Curtin-Hammett principle, the selectivity of reaction products relates to the free energy difference of stability at the transition state or structure in the selectivity-determining step. Therefore, it is a reasonable assumption that the solvent molecules in enzymatic reaction influence mainly the difference of conformational change of enzyme at the transition state. The conformational change (fluctuation) of enzyme protein arises from various interactions between enzyme and solvent such as electrostatics, hydrogen-bonding, cohesive pressure, hydrophobic interactions, etc. In this paper, a three-parameter treatment was proposed as a linear function of three complementary parameters describing the polarity, cohesive pressure, and hydrophobic factors of the given solvent. In this case, the Kirkwood parameter ［(-1)(2+1)］ was chosen as the polar factor, the square of solubility parameter 2 as the cohesive factor, and log P as the hydrophobic factor. According to the model, the logarithm of the product ratio (ln ) can be described in terms of equation ln =a［(-1)/(2+1)］+b2+clog P+d. This three-parameter equation has been applied to estimate the solvent effect on the selectivity of various enzymatic and organic reactions, and presents a high linearity with correlation coefficients in the range from 0925 to 0998. The proposed three-parameter analysis can be an extremely useful tool for the investigation of solvent effects on the selectivity in non-polar concerted organic reactions as well as enzymatic reactions.