Vapor pressure isotope effects (VPIEs) in monatomic systems (neon to xenon, either between pure isotopes or in their binary mixtures) were evaluated using an integral equation theory for a Lennard-Jones fluid with the Duh-Haymet-Handerson closure. The most relevant quantity obtained in this way is the average of the Laplacian of the potential energy of the system, , also known as the mean force constant. The results correctly predict the different rare-gas VPIEs which span over several orders of magnitude. Using a simple two-parameter corresponding states principle, the method is capable of predicting VPIEs simply from the knowledge of isotopically independent Lennard-Jones parameters of each rare gas and the masses of its isotopes. Each type of VPIE (in pure isotopes or mixtures) map onto two reduced variable equations in terms of ln(f(1)/f(g)(o))(*) and ln(gamma(infinity))(*). The former quantity represents a reduced form of the reduced partition function ratio (a measure of the VPIE between pure isotopes) while the second is a reduced form of the liquid activity coefficient at infinite dilution (a measure of VPIEs in isotopic binary mixtures). Several issues related to the temperature and density dependence of are also addressed in this work. (C) 2003 American Institute of Physics.