The transition from particle shearing to looping was investigated in single crystals by using optical microscopic, transmission electron microscopic and mechanical testing methods. Observations of slip-line morphology and of dislocation structures revealed the occurrence of a transition in deformation mode from particle shearing to by-passing via Orowan looping. Loop-size measurements suggested that the critical loop size was close to the average planar particle size at the onset of the peak-aging condition. On the basis of the transition behavior, the antiphase boundary energy was estimated to be about 100mJ/m2. This was lower than that deduced from dislocation pair spacings. A similar analysis of the aging curves of polycrystals which contained Zr tended to predict a higher antiphase boundary energy. This was because the peak-aging condition appeared relatively rapidly, due to the presence of a bimodal size distribution of ' particles in those crystals. The dislocation pair spacings decreased, in the early stages of aging, with increasing particle size. Transition Behaviour of Deformation Mode from Shearing to Looping in Al‒Li Single Crystals. S.M.Jeon, J.K.Park: Philosophical Magazine A, 1994, 70[3], 493-504