A study of the mechano-chemical transformation of goethite to hematite was carried out using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Synthetic goethite was dry-ground in air for different times up to 104 h and characterized with the aim of understanding the mechanisms taking part in the transformation, the phases formed, and particle distribution, morphology and size. It was found that the transformation is topotactic, without the formation of intermediate phases, and takes place by fragmentation of crystals, creation of twins, dehydration of goethite particles, and formation of voids. The voids are arranged in almost parallel striations, and are associated with the loss of hydroxyl groups and formation of hematite in small domains of equiaxial crystallites. The hematite crystal size is about 10 nm at the initial stages and after prolonged periods of grinding remains constant at about 19 nm.