Superplastic forming of AA5083 is typically limited by cavity formation and linkage rather than by strain localization associated with necking. Cavity formation during both biaxial and plane strain conditions in superplastically enabled AA5083 may occur in association with Al6Mn and Al3Fe particles that reside on boundaries, although some cavities are not obviously associated with particles. Measurement of grain-to-grain disorientations across cavities reveals that cavities form by grain boundary sliding on grain boundaries of disorientation ≥ 10º under both deformation conditions. For both biaxial and plane strain loading grain boundary sliding contributes to cavity formation and growth for solute-drag creep controlled deformation as well as for grain boundary sliding controlled deformation.