Field ionization of high-n CH3I Rydberg states doped into argon is presented as a function of argon number density along the critical isotherm. These data exhibit a decrease in the argon induced shift of the dopant ionization energy near the critical point. We show that this decrease is due to the interaction between argon and the quasi-free electron arising from field ionization of the dopant. The energy of the quasi-free electron in argon near the critical point is calculated in a local Wigner-Seitz model containing no adjustable parameters to within 0.2% of experiment. (c) 2005 Elsevier B.V. All rights reserved.