In situ measurements during the constrained sintering of Gd-doped ceria reveal tensile stresses up to ∼250 MPa. These large tensile stresses are likely to contribute to the reduced densification (compared with freely sintered material) typically observed during constrained sintering. While existing models postulate that the tensile stress in a densifying constrained film cannot exceed the “sintering stress,”σS, the observed tensile stresses are significantly larger than the estimated σS for these materials. To explain this observation, we propose that the formation and extension of interparticle grain boundaries induce substantial tensile stresses in constrained films. A model of this phenomenon shows that converting excess surface energy to elastic strain energy can produce stresses that are comparable to the measured values. Further, if these “cohesive” stresses exceed σS, grain-boundary diffusion should initially move material from the neck regions into the grain boundaries, not out of the grain boundaries as described by traditional sintering models.