The geometrical optimizations of various isomers of 4-methyl-3,6-pyridazinedione were carried out using MO computations at the B3LYP/6-31G(d), B3LYP/6-31+G(d,p) and MP2/6-31G(d) levels of theory. Nine isomers of the 4-methyl-3,6-pyridazinedione and 10 transition states of their transformations were found. Energetics, thermodynamic quantities, rate constants and equilibrium constants of the tautomeric and rotameric transformations of all isomers of the 4-methyl-3,6-pyridazinedione in gas phase and aqueous phase were obtained. (c) 2005 Elsevier B.V. All rights reserved.