Thermal and photochemical reactions of La-2@C-78 with 2-admantane-2,3-[3H]-diazirine are investigated. Four isomers of the monoadcluct (La-2@C(78)Ad) synthesized by the photoreaction are isolated by HPLC and characterized by mass, UV-vis-NIR absorption, cyclic voltammogram and differential pulse voltammogram spectroscopy, proton and C-13 NMR spectroscopic analysis, single-crystal X-ray diffraction analysis, and theoretical approaches. The addition reactions occur at both the [5,6] and [6,6] positions. X-ray and theoretical studies indicate that one of the monoadduct isomers has an open structure with two La atoms on the C-3 axis of the D-3h cage of La-2@C-78.