The structural evolution of natural brewsterite Sr1.30Ba0.66(H2O)10[Al4.00Si12.00O32], compressed in penetrating (water:ethanol 1:4 mixture) and non-penetrating (paraffin) media up to 3.5 GPa, was studied using single-crystal X-ray diffraction data from a diamond-anvil cell. The results clearly demonstrate the absence of pressure-induced hydration in the brewsterite structure. In both media, the behaviour of brewsterite under compression is similar. A nearly isotropic compression of brewsterite up to 2 GPa proceeds through a slight decrease of the framework T–O–T angles. At P > 2 GPa a phase transition occurs, which is evinced by an abrupt volume reduction by about 1%. The structure contracts mainly along the a axis and exhibits almost regular changes along the two other axes. The high-pressure phase is characterized by a doubled c parameter of the unit cell; the space group remains the same as for the initial phase.