The acidity constants of 5-uracilmethylphosphonic acid, H2(5Umpa), and 6-uracilmethylphosphonic acid, H2(6Umpa), were determined by potentiometric pH titrations in aqueous solution (25 °C; I = 0.1 M, NaNO3). Comparison of these constants with those of related uracil derivatives (partly taken from the literature) allows the conclusion that an intramolecular hydrogen bond is formed between (N1)H and the phosphonate group of 6Umpa2−; the formation degree of this hydrogen-bonded isomer is estimated to be 86 ± 7%. The X-ray crystal structure analysis of H2(6Umpa) is reported but this solid state structure is dominated by intermolecular hydrogen bonds. In the context of the properties of 5Umpa2− and 6Umpa2− those of uracil are also discussed and from various comparisons of acidity constants it is concluded that deprotonation of uracil may occur at (N3)H as well as at (N1)H but that the (N3)-deprotonated species dominates with about 80% in aqueous solution at 25 °C and I = 0.1 M (Na+). The search for other examples of uracil derivatives which allow hydrogen-bond formation in aqueous solution has led to orotic acid (= 6-uracilcarboxylic acid; [H(6Urca)]) and 5-uracilcarboxylic acid [H(5Urca)]; based on acidity constant comparisons it is concluded that in aqueous solution (25 °C; I = 0.1 M, KCl) H(5Urca) exists to about 92 ± 10% as a species with a hydrogen bond between (C5)COOH and (C4)O, and 6Urca− to about 95 ± 5% as a species with a hydrogen bond between (C6)COO− and (N1)H. The importance of intramolecular hydrogen-bond formation to the acid–base properties of compounds in solution is briefly emphasized.