[Er-2(thd)(4)Pc]center dot 2C(6)H(6) (1) (Hthd = 2,2,6,6-tetramethylheptanedione), obtained as green crystals from the reaction of [Er(thd)(3)]center dot 2H(2)O with lithium phthalocyanine, Li2Pc, is a stable dinuclear complex with two Er-III centers. Its lattice benzene solvent can be exchanged by soaking the crystals in dichloromethane to give [Er-2(thd)(4)Pc]-2CH(2)Cl(2) (2). The magnetic susceptibility data suggest different coupling interactions for the two complexes. While 1 exhibits fast relaxation and an estimated energy barrier of Ea = 2.6 cm(-1) under 600 Oe dc field, the single-molecule magnet behavior of 2 is field-induced and the energy barrier is higher at 34.3 cm(-1). Ab initio calculations were performed to understand the nature of the coupling interaction between two Er-III ions bridged by the phthalocyanine and the origin of different magnetic behavior. Importantly, the single-molecule magnetic properties can be reversibly tuned through the exchange of solvent molecules, confirmed by further measurements on the complexes 1-re and 2-re. This subtle control of relaxation by lattice solvents is rarely observed in single-molecule magnets, especially for Er-III-based complexes.