PRESSURE-induced amorphization of solids has been much studied since it was first observed in 1984(1). It was found recently(2,3) that some materials can be amorphized reversibly under pressure, reverting back to the original crystalline structure and orientation when the pressure is decreased. It has been suggested(4) that the presence of non-deformable units is essential for this reversibility, these units acting as templates around which the original structure is reformed. Here we investigate this idea by comparing the effect of pressure on two clathrasils-silica solids with open, microporous structures-with and without guest molecules inside the pores. We have studied pressure-induced amorphization of dodecasil-3C, which has cage-like voids, and dodecasil-3R, which has two-dimensional channels. For both materials, our experiments and simulations show that amorphization is fully or partly reversible only when guest molecules are present suggesting that these do indeed act as rigid ’organizing centres’ for the reversible transformation.