He-3 NMR spectroscopy has been used to study the equilibria of Diels-Alder additions of 9,10-dimethyl anthracene (DMA) to He-3@C-60 and He-3@C-70. Spectra of a series of equilibrium mixtures showed peaks for the isomeric adducts. One monoadduct, six bis-adducts, eleven tris-adducts, and ten tetrakis-adducts of DMA to C-60 were seen. One monoadduct and three bis-adducts of C-70 were detected. Equilibrium constants were found for these reactions and values for DeltaG, DeltaH, and DeltaS were obtained.