Three hydrogen bonds are formed between creatinine and derivatives of 2-amino-4(3H)pyrimidone. Changes in the tautomer ratio of the latter upon complexation can be observed by UV spectroscopy. The formation of a further hydrogen bond between creatinine and a 4-aminoacridin-5-yl-amino-substituted 4(3H)pyrimidone seems to be sterically prevented. The properties of three other, quite different compounds assumed to form three hydrogen bonds with creatinine have also been investigated. Dodecyl 4-octadecylallophanate and 2,6-bis(dodecylamino)-pyridine do not associate appreciably with creatinine, which can be explained by the formation of an intramolecular hydrogen bond in the former compound and the lack of tautomerization of the latter. With 6-(1-heptyloctylamino)-2(1H)-pyridone, however, creatinine forms a complex that is almost as stable as those with 2-amino-4(3H)-pyrimidone derivatives.