Field ionization measurements of trimethylamine in methane and triethylamine in ethane are presented as a function of perturber number density at various noncritical temperatures and near the perturber critical isotherm. Critical point effects are observed that are similar to those seen in atomic fluids. A two-Yukawa potential is shown to model the perturber/perturber interactions accurately by calculating the phase diagram of the perturber fluids, and using this potential the local Wigner-Seitz model for the energy of the excess electron is successfully applied to these molecular systems. (C) 2009 Elsevier B.V. All rights reserved.