Rietveld refinements using synchrotron powder X-ray diffraction data were used to study the crystal structure and dehydration behavior of pure monoclinic palygorskite samples from Korea and Alaska. The 300 and 100 K palygorskite structures in air compare well with previous models but provide additional details about zeolitic H2O sites and reveal that the Al atoms are ordered into the inner M2 octahedral sites and the Mg cations into the M3 sites at the edges of the tunnels. Real-time, temperature-resolved synchrotron powder X-ray diffraction data and Rietveld refinements were used to investigate the monoclinic palygorskite structure from 300 to 1400 K (in air). Rietveld refinements showed that most of the zeolitic H2O is lost by ~425 K, accompanied by a decrease in the unit-cell volume of 1.3%, primarily owing to a decrease in the a unit-cell parameter and an increase in the β angle. The structurally bound H2O is lost in two stages, at temperature intervals of 475–540 and 580–725 K. Above ~825 K in air a portion of the Korean sample transformed to a folded structure; the Alaskan sample folded at ~575 K under vacuum. A structure model was refined for the folded structure. At ~1015 K for the sample heated in air, β-quartz diffraction peaks appeared and increased in intensity as heating continued to the maximum temperature. Cristobalite formed above ~1050 K, along with a small amount of clinoenstatite, and both phases persisted to the maximum temperature studied.