The particular features of the v(OH) absorption band of dimers with a strong hydrogen bond (ΔH = 24-50 kcal/mol = 8000-17000 cm ^-1/molecule) formed by molecules of phosphinic acids R ₂POOH N₂ are studied in the gas phase, low-temperature argon and nitrogen matrices, and crystalline films. It is found that, irrespective of the type of the acid, the v(OH) IR absorption bands of dimers are broad (Δv_1/2 ∼ 1000 cm^-1) and similar in shape, exhibiting a characteristic ABC structure. The formation of these anomalously broad absorption bands is shown to be primarily associated with vibrations of the -POOH fragments, participating in the hydrogen bonding. A change in the temperature in the range 12-600 K and the passage from cyclic dimers in the gas phase to helical chains with hydrogen bonds in the crystalline state cause no significant changes in the shape, width, or structure of the dimer band. The contribution to the formation of the broad absorption band of the (R₂POOH)₂ dimers made by anharmonic interactions between the high-frequency v(OH) vibration and the low-frequency intermolecular vibrations is estimated. The absorption spectra of weak complexes R ₂POOH...N₂ in matrices at 12 K are discussed.