The solvent density dependence of the solvation energy difference associated with a charge transfer process in a polar dumbbell fluid is studied from the gaslike to the liquidlike densities by means of two types of the integral equation theories and the Monte Carlo simulation. The polar dumbbell fluid model including the Coulombic interaction explicitly has succeeded in a qualitative reproduction of quite a larger density dependence of the solvation energy in the low-density region than that in the higher-density region. The origin of the density dependence has been attributed to the difference in the reaction field response at various densities. At the low density, the reaction field shows a highly enhanced nonlinear response to the solute dipole moment. The nonlinear enhancement is an intrinsic nature in the gaseous dielectric solvation, and is due to the enhanced local density around the solute molecule with increasing the solute charge. On the other hand, the reaction field at the high density responds almost linearly to the solute dipole moment. The analysis based on the integral equation has clarified that the high linearity at the liquid density is closely related to the invariance of the radial distribution between cores in spite of the charging on the solute, which reflects the highly packed structure at the high density. When the solute dipole moment is quite large, we observe the dielectric saturation at the high density, where the enhanced local density around the solute does not result in the increase of the reaction field because of the saturation in the orientational correlation between the solute and solvent dipoles. Even when the solute dipole moment is small enough not to cause the nonlinear response, the reaction field is induced nonlinearly to the solvent density, which is interpreted not only by the local density but also by the contribution per solvent molecule at various densities. This study demonstrates that the following two properties often neglected so far have significant effect on the evaluation of the solvation energy difference at various densities: one is the nonlinearly enhanced field response at the gaseous density, and the other is the density variation of the reaction field induced per solvent molecule. (C) 2000 American Institute of Physics. [S0021-9606(00)50610-2].