The NMR spectra of solutions containing partially deuterated anhydrous hypophosphorous acid (H₂POOH) and its complexes with organic bases as proton acceptors were obtained in CD₂Cl₂ in the temperature range 183-253 K. Under these conditions, the state of slow exchange is achieved, as evidenced by the fine spin-spin and isotope (H/D) structure of the NMR signals. The formation and strengthening of the hydrogen bond by the OH group result in strong shielding of the ³¹P nucleus and decrease the spin-spin coupling constants of nuclei in the PH₂ group. Saturation of these effects occurs in going from proton to base. Direct and long-range effects of H/D substitution in the OH and PH groups on the H, ³¹P, and ¹⁵N chemical shifts in complexes were measured. The signs of these effects were explained in terms of a simplified model of dynamic interaction of covalent and hydrogen bonds. The kinetics of the interconversion of a cyclic H₂POOH dimer and a zwitterionic complex with pyridine were studied by dynamic ¹H NMR, and thermodynamic and kinetic parameters of the process were measured. A hypothetical mechanism of the reaction with the transition state close to an open-chain dimer with one hydrogen bond was proposed.