In order to gain a better understanding of the mechanisms that control recrystallization and nucleation processes three MgO single crystals (ps1, ps2 and ps3) were deformed in axial compression parallel to <100> in a 1 atmosphere creep rig at a temperature of 1400degreesC, stress of 32 to 35 MPa and up to 31% strain. Quasi steady-state strain rates of 10(-7) s(-1) were measured. All specimens deformed heterogeneously. The microstructures were investigated using automated electron backscatter diffraction mapping (EBSD). In all samples orientation maps of sections cut parallel to the loading direction, show continuous misorientation across the crystals from a reference orientation. The maximum deviation angle measured is 36degrees. While in ps2 (17% strain) very little substructure can be observed, sample ps1 (24% strain) and ps3 (31% strain) are characterised. by a network of low angle boundaries (1degrees to 5degrees) and incipient sub-grain boundaries of 5degrees to 10degrees. These can be interpreted to have formed by sub-grain rotation recrystallization assisted by climb of dislocations. Continuous small circle dispersion of the poles to {100} and {110} with [001] as the rotation axis, suggests {110}<110> as the most likely active slip systems, while slip on {100}<110> may be triggered by heterogeneous deformation.