During p-xylene(XYL) sorption in MFI type materials presenting various framework compositions at room temperature, the M (monoclinic, centric P2(1)/n . 1 . 1 group) --> O2 (orthorhombic, acentric P2(1)2(1)2(1) group) transition is never observed : for high Si/Al ratios (>approximate to 75) the M-->O1 is first observed at lower sorbate loadings (0 to 4 molecules/uc), and the O1 (orthorhombic, centric Pnma group) --> O2 transition is only achieved at situation (8 molecules/uc); in the case of low Si/Al ratios (<75), which is the usual situation (genuine ZSM-5 phases), only O1-->O2 is observed. On sorbate desorption, both O2-->O1 and O1-->M reversible transitions are observed. In the present work the deformations of the tenring (10R) channel-openings in the straight and zig-zag channels are investigated versus p-xylene loadings in silicalite (Si/Al approximate to 5000). It is shown that during the O1-->O2 phase transition occuring at higher sorbate loadings the mean/maximum displacements of framework oxygen atoms are close to approximate to 0.57/1.03 Angstrom, which is significantly higher than the mean displacements calculated for all the framework atoms(Si and O) in a theoretical (molecular mechanics) study (0.36 Angstrom) during the hypothetical and unobserved M-->O2 transition. It is shown that during the three-step p-xylene sorption mechanism the framework is highly flexible. The effective pore-openings in silicalite . 4XYL are 6.16x4.78 and 5.65x5.25 Angstrom for the straight and zig-zag channels respectively.