Field ionization has been employed to determine the ionization potential of CH3I doped in krypton for krypton densities between the gas and the triple point liquid. The ionization potential is correlated to the different configurations of the host atoms around the dopant at the moment of excitation and is, therefore, represented by a distribution. The ion-medium polarization energy distribution was calculated, from the first and second moments, at the densities at which the experiments were performed. The calculated widths are in very good agreement with those of the experimentally determined ionization potentials. By combining the experimental results with those of the calculations, the density dependence of the quasi-free electron energy in Kr was determined. These results are compared to those obtained in previous experiments and with recent theoretical calculations.