Summary. The synthesis of the title compound 7 was achieved starting with mucochloric acid via the pyridazinones 1, 2 and 6. The electrolytic ionisation constants for 7 were found to be 4.81 (_+ 0.03) and - 0.3 (_+ 0.3). Crystal structure analyses were performed for 7 and its "fixed" derivatives 6 and 9. ... Keywords. Synthesis of 5-methoxy- and 5-hydroxy-3(2H)-pyridazinones; pK, Values of 5-hydroxy- 3(2H)-pyridazinone; Crystal structure of 3(2//)-pyridazinones.