Reaction of the bicyclic phosphazane N₅P₄Et₅Cl₂ with 2,6-dimethylphenol and subsequent oxidation of the product by aqueous hydrogen peroxide yields N₅P₄Et₅O₄(OC₆H₃Me₂-2,6)₂ in 85% yield. Its structure has been established by NMR spectroscopy and single-crystal X-ray diffraction. The compound crystallises in the monoclinic space group C2/c with a= 21.245(5), b= 10.879(2), c= 16.450(6)Å, β= 123.94(2)°, Z= 4, R= 0.066. The structural features are compared with those of bicyclic λ⁵-phosphazenes of type N₅P₄R³(NR¹R²)₅(NHR³)(R¹,R³= Me or Et, R²= H or Me). The observed conformation of the N₃P₃ rings in the present compound is mainly dictated by the maximisation of the stabilising influence of ‘negative hyperconjugative interactions’ between the nitrogen lone pairs and the adjacent P–X σ* orbitals.