(81)Br NQR measurements of the mixed-valence compound (NH(4))(4)Sb(III) Sb(V)Br(12), which has Sb(V)Br(6)(-) and Sb(III)Br(6)(3-) octahedra in different oxidation states in the tetragonal pseudo-K(2)PtCl(6) structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T(c) = 212 K was redetermined. The T(1) values are quite different between Sb(V)Br(6)(-) and Sb(III)Br(6)(3-) octahedra. For all resonance lines T(1) minima were observed at Tc. The T(1) behavior at T(c) was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both (81)Br NQR frequencies and T1 values for each anion indicates that the static rotation may occur for Sb(V)Br(6)(-) but not for Sb(III)Br(6)(3-) 6 at T(c) in the low temperature phase.