The thermodynamics for intermolecular hydroamination of vinylarenes with arylamines have been measured directly by conducting the addition processes, as well as cleavage of the addition products, under conditions in which amine, vinylarene, and phenethylamine are all present. The reaction of N-methylaniline with styrene is exothermic by about 10 kcal/mol but nearly thermoneutral in free energy. The free energies for additions of various primary arylamines to styrene and for additions of primary arylamines to indene, dihydronaphthalene, and two vinylarenes range from 1.3 to -3.5 kcal/mol (K = 0.16-155 M-1). The steric properties of the nucleophile significantly influenced the equilibrium constant for addition, but the electronic properties of the nucleophile had a minor effect on this equilibrium constant. These measurements have led to the first successful intermolecular addition of aniline to indene and 1,2-dihydronaphthalene and shed light on factors requiring consideration when choosing substrates and reaction conditions for this transformation.