In chemical terms, creatinine is a lactam and an imidazolidinone, so a spontaneously formed cyclic derivative of creatine.
Several tautomers of creatinine exist; ordered by contribution, they are:
*2-Amino-1-methyl-1H-imidazol-4-ol (or 2-amino-1-methylimidazol-4-ol)
*2-Amino-1-methyl-4,5-dihydro-1H-imidazol-4-one
*2-Imino-1-methyl-2,3-dihydro-1H-imidazol-4-ol (or 2-imino-1-methyl-3H-imidazol-4-ol)
*2-Imino-1-methylimidazolidin-4-one
*2-Imino-1-methyl-2,5-dihydro-1H-imidazol-4-ol (or 2-imino-1-methyl-5H-imidazol-4-ol)
Creatinine starts to decompose around 300 °C.