The flexibility and structure transition behaviour of ZIF-8 in a series of samples with different particle size has been studied using a combination of high-resolution N2 gas adsorption isotherms and, for the first time, a broad in situ PXRD and Rietveld analysis. During the stepped adsorption process, large particles showed a narrow adsorption/desorption pressure range with a shorter equilibrium time due to lower kinetic hindrance, deriving from higher amount of active sites. In situ PXRD showed that both the rotation of imidazole ring and a bend in the methyl group led to the gate opening of ZIF-8. ; This work was funded by the EPSRC IAA Partnership Development Award (RG/75759). D. F.-J. thanks the Royal Society for funding through a University Research Fellowship. We thank Diamond Light Source for beamtime at beamline I11 (visit EE9750). ; Peer reviewed