Experiments were performed to visualize, in situ, the deformation and leakage of soft seals under differential pressure. Two annular seals and a flat, rectangular seal were investigated in the experiments. The observations show that large, nonuniform elastic deformation precedes seal leakage. A corresponding finite element model for a rectangular solid seal shows that large deformation leads to a decrease in the compressive sealing stress between the seal and the surface against which it seals. The predicted flow path for the model is qualitatively similar to the flow path observed in the experiments. A leakage mechanism in which nonuniform elastic deformation leads to seal leakage is proposed.