Abstract Partition coefficients (KD) of benzene and 11 chloro derivatives of benzene were determined at 25 °C in two liquid-liquid partition systems: heptane/water and heptane/acetonitrile. Increase of KD with increase of the numbers of Cl atoms in the solute is discussed in terms of surface and cohesive energies. The small difference in KD of the isomers whose Cl atom numbers are 2, 3, or 4 was correlated with a difference in the dipole moment of the solute.