The structural properties of the Na 2 CsC 60 fulleride have been studied by synchrotron X-ray powder di4raction at both ambient and elevated pressures. Complementary neutron di4raction measurements at high pressure were also performed. We 5nd no evidence for a monomerPpolymer phase transition on cooling at ambient pressure, despite the adopted slow cooling procedures, with the structure remaining strictly cubic, even after prolonged standing at 200 K. The pressure dependence of the structure of solid Na 2 CsC 60 at ambient temperature was followed up to 0.56 GPa by neutron di4raction and up to 8.63 GPa by synchrotron X-ray di4raction. At ambient pressure, the structure is primitive cubic with a)3(9231.41؍ A s (space group Pa3 ). When a pressure of 0.76 GPa is reached, an incomplete phase transition to a lowsymmetry structure, accompanied by a large volume decrease (2.7(1)%), is encountered. This phase was characterized as monoclinic with a)5(547.31؍ A s , b)6(422.41؍ A s , c)3(804.9؍ A s , and 3)1(17.331؍ (space group P2 1 /a), isostructural with the low-temperature polymer phase of Na 2 RbC 60 . The cubic and polymeric phases of Na 2 CsC 60 coexist up to 0.90 GPa. The pressure evolution of the monoclinic lattice constants a, b, and c to 8.63 GPa reveals the presence of substantial anisotropy in the compressibility along the three axes. The structure is least compressible along the c axis, which de5nes the polymeric C+C bridged fulleride chains and is most compressible along the interchain b direction.