The aim of this work is to investigate the dielectric polarization in polar-molecule reactions in the liquid phase theoretically. On the basis of the modified Smoluchowski equation, the analytic expression of the polarization is derived. The result shows that the expression of the polarization in polar-molecule reactions is quite different from the one in the Debye model. For general reactions, two vectors, the rotational diffusion vector and the component concentration vector, are proposed to describe the polarization. For fast reactions, the conception of relative permittivity will be nonexistent due to the effect of the component concentration vector on the frequency of the applied field, and the dielectric behavior may only be analyzed in the time domain. On the contrary, for slow reactions, the relative permittivity will be concentration-dependent.