| Summary: | The self-association of phosphonic acids with general formula RP(O)(OH)<sub>2</sub> in solution state remains largely unexplored. The general understanding is that such molecules form multiple intermolecular hydrogen bonds, but the stoichiometry of self-associates and the bonding motifs are unclear. In this work, we report the results of the study of self-association of <i>tert</i>-butylphosphonic acid using low temperature liquid-state <sup>1</sup>H and <sup>31</sup>P NMR spectroscopy (100 K; CDF<sub>3</sub>/CDF<sub>2</sub>Cl) and density functional theory (DFT) calculations. For the first time, we demonstrate conclusively that polar aprotic medium <i>tert</i>-butylphosphonic acid forms highly symmetric cage-like tetramers held by eight OHO hydrogen bonds, which makes the complex quite stable. In these associates. each phosphonic acid molecule is bonded to three other molecules by forming two hydrogen bonds as proton donor and two hydrogen bonds as proton acceptor. Though the structure of such cage-like tetramers is close to tetrahedral, the formal symmetry of the self-associate is C<sub>2</sub>.
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