Two series of 1:1 complexes with strong OHN hydrogen bonds formed by dimethylphosphinic and phenylphosphinic acids with 10 substituted pyridines were studied experimentally by liquid state NMR spectroscopy at 100 K in solution in a low-freezing polar aprotic solvent mixture CDF ₃ /CDClF ₂ . The hydrogen bond geometries were estimated using previously established correlations linking ¹ H NMR chemical shifts of bridging protons with the O···H and H···N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of ³¹ P NMR signal upon complexation. We show that the values of ³¹ P NMR chemical shifts are affected by an additional CH···O hydrogen bond formed between the P?O group of the acid and ortho-CH proton of the substituted pyridines. Breaking of this bond in the case of 2,6-disubstituted bases shifts the ³¹ P NMR signal by ca. 1.5 ppm to the high field.