Polymerization by acid-induced aggregation, a process that was previously unknown in organometallic chemistry, is shown to occur in methyltrioxorhenium(VII). The resulting polymer, an oxide ceramic precursor, undergoes an unusual depolymerization: in the weeks following compression of a pellet of the polymer, the surface becomes covered with large single crystals of the monomer (see figure). This is the first example of the specific crystallization of a metal-organic compound from the solid state by application of pressure.