Highly crystalline silicalite ( Si Al >5000 ) obtained by hydrothermal dealumination of a ZSM-5 material ( Si Al =132 ) has been investigated at several temperatures by X-ray powder diffraction. Interpretation of the step-scanned XRD profiles by the Rietveld structure refinement code (DBW) shows that the displacive P2 1 n·1·1( M ) ⇆ Pnma ( O1) polymorphic phase transition occurs between 77 and 90°C (≈83°C). On heating (20–127°C temperature range) the minor and major axis-directions of the elliptical D10R (double ten-ring) pore-openings corresponding to the straight-channels ( 8.50×7.95 and 8.55×7.98 A ̊ at 20°C) progressively change and are finally inverted ( 7.99× 8.52 A ̊ at 90°C). At the same time, the elliptical D10R sections of the zig-zag channels ( 8.68×8.06 and 8.51×7.69 A ̊ at 20°C) become more circular ( 8.35×8.13 and 8.43×7.96 A ̊ at 90°C).