Ab initio calculations have been used to study the intrinsic basicity and acidity of hypofluorous acid (FOH), which plays a key role as an intermediate in the interaction of Fz with water. At the G2 level of theory, the proton affinity and deprotonation enthalpy of FOH were predicted to be 135.3 and 360.3 kcal/mol at 298 K, respectively. In particular, the latter value is in excellent agreement with the best available thermodynamically calculated one. The computational results show that FOH is a significantly stronger acid (by ca. 24 kcal/ mel) and a weaker base (by ca. 30 kcal/mol) than H2O. The influence of the replacement of one hydrogen atom in water molecule with F atom on its acid-base properties is rationalized in terms of field-inductive, polarizability, resonance, and adjacent lone pair-lone pair interaction effects.