Here we present detailed structural data on the effect of high pressure on Cu-btc. Inital application of pressure (0.5 GPa), causes the pores and unit cell volume to expand as solvent is squeezed into the pores. Increasing pressure further causes more solvent to enter the pores, until at 5.0 GPa, a phase transition takes place. The transition is driven by the sudden compression and expansion of equatorial and axial Cu-O bonds, as we change from a pore filling to a pore emptying mechanism.