The instability of supported poly(methyl methacrylate) (PMMA) thin films in water has been investigated. It is found that PMMA films partially detach from the solid substrate, resulting in the formation of bubbles under water. The process is reversible. Surface morphology analysis shows that the radius of curvature of the bubbles is dependent on the thickness of the PM M A films and is independent of the treatment of the films, such as the annealing temperature and the annealing time. Theoretical analysis based on a two-layer model (the swollen layer and the interior layer) shows that the partial swelling of PM MA in water is the physical origin of bubble formation.