It has been recently claimed that the large cohesive energy density of water is the ultimate cause of the poor solubility of nonpolar compounds in water. In order to test the validity of this idea, we analyze the difference in solubility between light water and heavy water of several nonpolar compounds. Even though the cohesive energy density of D2O is larger than that of H2O, nonpolar compounds are slightly more soluble in D2O than H2O. In such case experimental data do not support the correctness of the large cohesive energy density as the ultimate cause of hydrophobicity. We show that D2O is a slightly better solvent than H2O for nonpolar compounds because it is slightly less costly to create a cavity in the former liquid. This is because there is slightly more void volume in heavy water than in light water. (C) 2004 American Institute of Physics.