A large increase in lipophilicity is observed for CH2/CF2 replacement in the center of di-functional compounds. The increase, in the range of 0.6–1.4 units is for most cases, far above expected values. The effect is nearly additive, thus one can switch from a hydrophilic compound to a lipophilic one using a limited number of H/F exchanges. In this paper, the effect and its origins are explored.