Tris(bromomethyl)phosphineoxide and 2-[(2-[( Z)-1-(2-hydroxyphenyl)methylidene]aminophenyl)imino]methylphenol have been utilised as precursors in the synthesis of novel pentaco-ordinate dioxadiazaphosphacyclopentadecines[e.g. 3-[(2-hydroxyethoxy)methyl]-2,4-dihydro-12 H-3λ5-tribenzo[ f, j, n][1,5,9,12,3]dioxadiazaphosphacyclopentadecine-3-selone] in moderate (overall) yields. The synthesised compounds were characterised by elemental and spectral (IR, 1H, 13C, 31P NMR and MS) studies and their anti-microbial activity was evaluated.