Time-resolved synchrotron X-ray powder diffraction data were used to investigate the dehydration behavior of the chalcophanite (ZnMn3O7·3H2O) structure from 300 to 1060 K. Rietveld refinements revealed two obvious phase changes, at ~450 and ~950 K, corresponding to the dehydration of chalcophanite followed by transformation to a spinel structure (Mn-hetaerolite). Only small changes were observed in the chalcophanite unit cell from RT to ~438 K; the volume increased by ~0.8%, mostly caused by thermal expansion of ~0.5% along c. Above ~427 K, the interlayer water molecules were lost, resulting in a collapse of the interlayer spacing from ~7 to ~4.8 Å. The newly formed anhydrous phase (ZnMn3O7) retained chalcophanite’s R3̄ space group and a dimension, but the c dimension decreased from ~21 to 14.3 Å, and the Zn coordination changed from octahedral to tetrahedral. Above ~775 K the anhydrous chalcophanite began to transform to a spinel structure, corresponding to a Mn-rich hetaerolite [(Zn0.75Mn2+0.25)Mn23+O4]. By ~973 K the transformation was complete. The diffraction patterns did not show a significant increase in background during the transformation, indicating that the reaction did not involve transient amorphization. The phase change was likely triggered by loss of 1.25 of seven O atoms in the original anhydrous chalcophanite structure with a corresponding reduction of Mn4+ to Mn3+ and Mn2+.