Molecular exclusion surfaces [J. Bentley, J. Phys. Chem. A 104, 9630 (2000)] have been determined for a number of molecules and molecular ions interacting with argon or helium atoms. These surfaces represent the boundaries between the molecules and their environment and have application, for instance, to continuum solvation models. To make these surfaces useful for such applications, we present some simple models for exclusion surfaces which depend only on properties of the electron densities of the isolated molecules. (C) 2003 American Institute of Physics.