A combination of high pressure single crystal X-ray diffraction and high pressure SQUID magnetometry has been used to study three hydroxo-bridged copper(II) dimers. [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (1; tmen = tetramethylethylenediamine), [Cu2(OH)2(tben)2](ClO4)2 (2; tben = di-tbutylethylenediamine) and [Cu2(OH)2(bpy)2](BF4)2 (3; bpy = 2,2'-bipyridine) have been structurally determined to 2.5, 0.9 and 4.7 GPa, respectively. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of all three complexes. This is particularly true of the bond distances and angles between the metal centres and the bridging hydroxo groups. Compound 1 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in changes in the magnetic behaviour of the complexes as seen in dc susceptibility measurements up to approximately 0.9 GPa for 1, 2 and 3: the exchange becomes less antiferromagnetic in 1 and 2 and more ferromagnetic in 3.