An electron microscope study of the olivine to spinel transformation in Ni2SiO4 has revealed the nature of the defect structures in the transforming olivine immediately ahead of the migrating olivine-spinel interphase boundary. The residual olivine has a high density of [100> and [001> Burger's-vector dislocations (1 to 5¿109 cm-2). No stacking faults or dissociated dislocations are observed in the residual olivine, although a significant number (~108 cm-2) of [010> dislocations are activated by the extreme synthesis conditions (52 kbar, 1100 ¿C). This result may have important applications in the study of paleo-deformation of olivine. The mechanism of the transformation in the fine-ground sample appears to be a nucleation and growth process with no evidence to support a martensitic mechanism.