{Dy5(EDDC)2(μ3-AcO)2(μ5-C15H11PO3)(μ4-C15H11PO3)(μ2-AcO)2(AcO)2(H2O)(CH3OH)2}2(μ4-C2O4)·xH2O (I), where H2EDDC is N',N'',E,N',N'',E-N',N''-(ethane-1,2-diylidene)dipyrazine-2-carbohydrazide and C15H11PO3H2 is 9-anthrylmethylphosphonic acid, is found to undergo two consecutive single-crystal-to-single-crystal transformations. The first is under UV irradiation (λ = 365 nm for 3 d in air) to {Dy5(EDDC)2(μ3-AcO)2(μ5-C15H11PO3)2(μ2-AcO)2(AcO)2(H2O)3}2(μ4-C2O4)·xH2O (I-UV) where the two CH3OH are replaced by two H2O and the second by annealing under N2 at 100 °C on a diffractometer or under Ar in a glovebox to {Dy5(EDDC)2(μ3-AcO)2(μ5-C15H11PO3)2(μ2-AcO)4(H2O)}2(μ4-C2O4) (I-A-N2 or I-A-Ar) where it has lost two H2O molecules. The second transformation is reversible by exposure to air at room temperature (I-A-N2-cool). While the overall structures are the same (retaining the space group P21/c), there is a considerable expansion of the unit cell from I (8171 Å3) to I-UV (8609 Å3) and I-A-N2 (8610 Å3) and the coordinations of the Dy atoms undergo major reconstructions. This is associated with switching the single-molecule-magnetism (SMM) from OFF for I to ON for I-UV and to OFF again for I-A-Ar in air. Such a switching mechanism associated with the retention of crystallinity is unique in the chemistry of dysprosium. The structure of the molecule is formed from two symmetry related pentamers joined by an oxalate. A related compound containing two isolated neutral pentamers {Dy5(EDDC)2(μ3-AcO)2(μ5-C15H11PO3)2(μ2-AcO)3(AcO)2(H2O)2}{Dy5(EDDC)2(μ3-AcO)2(μ5-C15H11PO3)(μ4-C15H11PO3)(μ2-AcO)3(AcO)2(CH3OH)2}·2CHCl3 (II) has also been isolated with closely related Dy coordination and it exhibits similar SMM behaviour in zero field.