The high-pressure elastic properties behavior, phase stability, and mechanisms of amorphization of the alumino-silicate 7:4 mullite and a corresponding mullite doped with boron were investigated in situ by powder synchrotron X-ray diffraction with a diamond anvil cell in quasi-hydrostatic conditions. The samples of 7Al(2)O(3):4SiO(2) (Al4.66Si1.33O9.67), referred to as 7:4 mullite and an alumino-silicate mullite with 3.5(4) mol% B2O3, referred to as B-mullite, were compressed, in small pressure steps, up to 27.8 and 28.9GPa, respectively, and then decompressed back to ambient pressure. All along the compression path both samples' patterns are indexable with a mullite structure. Compression data are smooth up to a threshold pressure, from which point the diffraction peaks appeared to broaden, and the refined unit cell parameters deviate significantly down from the compressional trend. Above similar to 23GPa the diffraction patterns are not indexable anymore, suggesting amorphization. Rietveld structural refinements allow for a description of the pressure-induced main deformation mechanisms and structural trends. Pressure-induced mechanisms of amorphization are also discussed. A third-order Birch Murnaghan equation of state is fitted to the pressure-volume data to obtain experimental bulk moduli, as well as axial compressibilities for 7:4 mullite and B-mullite. Finally the volume compression in response to the applied pressure, combined with thermal expansion coefficients, allows a P-T-V equation-of-state for B-mullite to be proposed.