As a function of temperature the nitrite ions in sodium oxide nitrite, Na 3 ONO 2 , show unique and complex order/disorder phenomena. The crystal structures of the three low-temperature modi5cations of Na 3 ONO 2 have been determined by neutron powder di4raction. In the cubic room-temperature modi5cation (H-Na 3 ONO 2 , space group Pm3 m) the nitrite ions exhibit a 12fold orientational disorder. Upon cooling, the orientational disorder of the nitrite ions is reduced by two continuous order processes and one discontinuous phase transition. At T+ +240 K, H-Na 3 ONO 2 transforms by a continuous phase transition to tetragonal M-Na 3 ONO 2 (space group I4/mcm, a ؍ 6.49174(12) A s , c ؍ 9.19415(59) at T ؍ 195 K), and the orientational disorder of the nitrite ions is reduced via a continuous order process. The order parameter Q follows the power law Q+ + (T C ؊T) with T C ؍ 239.5(6) K and ؍ 0.28(2). At T ؍ 178 K a discontinuous phase transition ( H ؍ 620 J/mol) takes place and M-Na 3 ONO 2 transforms to T-Na 3 ONO 2 (space group P4/mbm, a ؍ 6.46513(7) A s , c ؍ 4.62382(6) at T ؍ 125 K). In T-Na 3 ONO 2 the nitrite ions exhibit a 2-fold orientational disorder. At T+ +68 K a second continuous phase transition to TT-Na 3 ONO 2 (space group P4 2 1 m, a ؍ 6.45345(6) A s , c ؍ 4.61973(6) at T ؍ 5 K) takes place and the orientational disorder is further reduced. This order process is characterized by a critical temperature of T C ؍ 67.4(2) K and a critical exponent of ؍ 0.21(1).