A simple theory is formulated in which the pressure of bubbles containing hydrogen molecules in the implanted region drives exfoliation due to the anisotropy of the elastic forces and the evaporation or slow diffusion of atomic hydrogen to molecular hydrogen in the bubbles leading to a catastrophic growth in the bubble radius and the molecular fraction as temperature increases. The process of blistering and cratering at small implant depths is also considered. Comparison to recent experiments is made. ©1997 American Institute of Physics.