The lambda(3)-cyclotriphosphazanes, [EtNP(OR)](3) [R = 2,6-Me(2)C(6)H(3) (1), 4-BrC(6)H(4) (2), or CH(2)CF(3)(3)], on treatment with tetrachloro-1,2-benzoquinone (TCB) give the lambda(5)-cyclodiphosphazanes, [EtNP(O(2)C(6)Cl(4))(OR)][EtNP(O(2)C(6)Cl(4)){N(Et)P(OR)(2)}] (5-7) by an unusual ring contraction-rearrangement. The reaction of the mixed substituent lambda(3)-cyclotriphosphazane, [(EtN)(3)P(3)(OR)(2)(OR')] [R = 2,6-Me(2)C(6)H(3), R' = 4-BrC(6)H(4)] (4), with TCB gives the lambda(5)-cyclodiphosphazane, [EtNP(O(2)C(6)Cl(4))(OR')][EtNP(O(2)C(6)Cl(4)){N(Et)P(OR)(2)}] (8), in which 4-bromophenoxide resides on one of the ring phosphorus atoms. The lambda(3)-bicyclic tetraphosphapentazane, (EtN)(5)P(4)(OPh)(2), on treatment with TCB undergoes a double ring contraction-rearrangement to give the lambda(5)-cyclodiphosphazane, (EtN)[(EtN)(2)P(2)(O(2)C(6)Cl(4))(2)(OPh)](2) (9). Variable-temperature and high-field (31)P NMR studies indicate the presence of more than one isomer in solution for the rearranged products 5-9. The solid state structure of 8 reveals a trans arrangement of the substituents with respect to the P(2)N(2) ring in contrast to the gauche arrangement observed for 5.