The phenylphosphonate and phenylarsonates Me2Sn[PhPO3], Me2Sn[PhAsO3], and Bun2Sn[PhAsO3] are isolated as their α-modifications from the reaction of dimethyl- or di-n-butyl-tin dichloride with the monosodium salt of the appropriate acid. β-Modifications are obtained by removal of water from the monohydrates Me2Sn[PhPO3]·H2O, Me2Sn[PhAsO3]·H2O, and Bun2Sn[PhAsO3]·H2O, obtained in turn from the reaction of the diorganotin dichloride with the disodium salt of the appropriate acid. Only one form of Ph2Sn[PhPO3] was isolated; this can be obtained either from the direct reaction of diphenyltin dichloride with phenylphosphonic acid or by removal of water from Ph2Sn[PhPO3]·H2O. On the basis of i.r. and variable-temperature Mossbauer spectroscopic and X-ray powder diffraction data it is suggested that the structures of the β-modifications and of Ph2Sn[PhPO3] consist of infinite chains, and that chains with similar backbone structures occur in solid-state forms of their hydrate precursors. The α-modifications of Me2Sn[PhAsO3] and Bun2Sn[PhAsO3] appear to have two-dimensional sheet structures in which tin achieves five-co-ordination, but the data do not preclude the possibility that α-Me2Sn[PhPO3] has a more associated three-dimensional network structure, with tin in highly distorted octahedral sites. The complexes Me2Sn[PhAs(OH)O2]2, Bun2Sn[PhAs(OH)O2]2, and Bun2Sn[PhP(OH)O2]2, obtained by the reaction of a diorganotin dihalide with the appropriate acid, have structures in which tin is in octahedral sites. It is suggested that these are chain structures in which chains are linked by strong hydrogen bonds.