The calcination of the zeolite ZSM-5 was studied by in situ X-ray synchrotron powder diffraction. Two samples were studied: One conventionally synthesized (CONV) and one synthesized in a charcoal matrix according to the confined space synthesis method (CSS). The powder diffraction data were collected between 375 K and 975 K in steps of approximately 8 K/5 min. and evaluated by Rietveld refinements. Both sets of powder diffraction patterns displayed the characteristic intensity changes for calcination, but neither of them showed the expected orthorhombic to monoclinic phase transition. Refinements of the extra-framework occupancy showed that the calcination starts at about 475 K and is essentially completed at 675 K for CSS, while the CONV calcination starts at 525 K and ends at 650 K. The refinement results agree very well with the TGA results. The carbon matrix of the CSS sample is combusted between 700 and 850 K. The unit cell dimensions continuously decrease from 475 to 975 K. An initial slow 1.2 % reduction between 375 and 875 K is followed by a fast 1.2 % reduction between 875 and 950 K for the CSS sample. This second unit cell reduction step is unrelated to the calcination process and does not involve any weight loss. It is suggested that this step is a recrystallisation/annealing process reducing the amount of defects and strain.