Wide-angle X-ray scattering measurements were performed to record structural changes during the transition from trehalose dihydrate to crystalline anhydrous alpha-trehalose. The results show that large dihydrate crystals rearrange into smaller sized alpha crystals; from the peak widths we calculate a crystallite size of typically approximately 40 trehalose molecules. We find that the dehydration probably takes place in a two-step process with different time scales for both the water removal step and the molecule rearrangement step. This suggests that there is crystal rearrangement in the dry state some 60 degrees C below the dry glass transition temperature of trehalose, which is unusual for a relatively large and strongly interacting molecule.