A study of a portion of the system NazO-CaO-A\zOa-SiOz has been made as a step in the solution of the problem of the state of combination of NazO in portland cement clinker. Quenching data are given for compositions in the tetrahedron formed by 2CaO.Si02, CaO, NazO.AlzOa, and AIzOa, and phase diagrams are shown for three composition planes through the space model. Five invariant points were located approximately, at one of which 2CaO.SiOz, 3CaO.SiOz, 3CaO.AIz03, and NazO.8CaO.3AI20a are in equilibrium with liquid. This point, which is not a eutectic, occurs at 1,440° ± 10°C and has a composition of 3.5 percent NazO, 55.2 percent CaO, 31.0 p ercent AbOa, and 10.3 percent Si02• Evidence was obtained of the formation, under favorable conditions, of a solid solution between 3CaO.AI20 a and NazO.8CaO.3AI203 at temperatures below that of liquid formation. Present information indicates that N a20 may be present in clinker in several possible forms: (1) in glass, (2) as Na20.8CaO.3AIzOa or a solid solution of this compound and 3CaO.AIzOa, (3) in solid solution in 2CaO.Si02, (4) as inclusions of a soda-bearing phase in ,8-2 CaO.Si02 produced by ex-solution on inversion from a-2CaO.Si02.