Hydrogen bond geometries in the proton-bound homodimers of ortho-unsubstituted and ortho-methylsubstituted pyridine derivatives in aprotic polar solution were estimated using experimental NMR data. Within the series of homodimers studied the hydrogen bond lengths depend on the proton affinity of pyridines and-at least for the ortho-methylsubstituted pyridines-on the pK(a) of the conjugate acids in an approximately quadratic manner. The shortest possible hydrogen bond in the homodimers of ortho-unsubstituted pyridines is characterized by the N center dot center dot center dot N distance of 2.613 angstrom. Steric repulsion between the methyl groups of the ortho-methylsubstituted pyridines becomes operative at an N center dot center dot center dot N distance of similar to 2.7 angstrom and limits the closest approach to 2.665 angstrom.