The siliceous zeolite Socony Mobil-twelve (ZSM-12) or MTW (Mobil-TWelve) with a one-dimensional pore system was studied at high pressure by synchrotron X-ray powder diffraction in nonpenetrating DAPHNE7474 oil and in a penetrating argon pressure medium. A phase transition from the space group C2/c to P2/n is observed in the nonpenetrating medium at close to 1.5 GPa with a 4% volume decrease and a strong increase in compressibility in the ac plane corresponding to partial collapse of the pores. Strong decreases in diffracted intensity are observed with further compression, and the diffraction pattern contains broad features characteristic of an amorphous material above 10 GPa. Distinct behavior is observed when this material is pressurized in argon. Argon fills the pores with 9 ± 1 Ar atoms per unit cell. In this medium-pore zeolite, the quantity of inserted argon was not found to vary with pressure. Argon-filled MTW is 35% less compressible than the corresponding empty form