AbstractWe present the first experimental measurement of the hardening arising from the detwinning of a single twin boundary in Mg. Microcompression tests were performed on two sets of microcolumns: (i) single crystals having a “parent” grain orientation of nearly (0 0 0 1) along the compression axis, and (ii) bicrystals involving the parent grain and a single twin boundary. A comparison of the stress-strain data shows significant differences in the deformation characteristics of the deformed microcolumns. The bicrystalline microcolumns undergo detwinning, as indicated by a stress plateau, which leads to a nominally single crystalline microcolumn of the orientation of the parent grain. Microcompression beyond the plateau shows that the detwinned microcolumns exhibit a considerably higher yield stress and strain hardening rate than the single crystalline “parent” microcolumns. Electron back-scattered diffraction analyses on the cross-sections of the deformed microcolumns reveal higher misorientations in the detwinned region indicative of a high content of unpaired dislocations (so-called GNDs), compared to the deformed parent microcolumns. This discrepancy in misorientation distribution between the samples is consistent with a detwinning-mediated hardening response, as observed, and points to the creation of dislocation debris as a consequence of the detwinning process.