The reactions of 2-trans-6-N4P4(NHPr n)2Cl6 (2), which was obtained from N 4P4Cl8 (1) and n-propylamine, with pyrrolidine and t-butylamine in different solvents have been studied. Compound (2) gave two different products, namely monocyclic (3 and 5) and bicyclic (4 and 6) phosphazenes. Compounds (2-6) have been characterized by elemental analysis, IR, 1H-, 13C-, 31P NMR, HETCOR and MS and the structure of compound (5) has been examined crystallographically. The bicyclic phosphazene (6) is the first exciting example of bicyclic phosphazenes containing chlorine atoms, in the literature. The formation mechanisms of bicyclic phosphazenes are re-considered by taking into account the synthesis of compound (6), which contains three stereogenic phosphorus atoms. Compound (5) crystallizes in the monocyclic space group P21/n with a = 13.974(2), b = 17.836(5), and c = 18.683(4) Å, β = 98.50(1)°, V = 4605.4(2) Å3, Z = 4 and Dx = 1.051 g cm-3. It consists of a non-centrosymmetric, non-planar phosphazene ring in a saddle conformation, with two n-propylamino (in 2-trans-6 positions) and six bulky t-butylamino side groups. The bulky substituents are instrumental in determining the molecular geometry. © 2003 Elsevier B.V. All rights reserved.