Abstract Thiocarboxylic acids smoothly attack conjugated azoalkenes at room temperature to give hydrazone 1,4-adducts that exhibit tautomerism with the corresponding enamino forms. These adducts when treated with sodium hydride in tetrahydrofuran at room temperature lead to 1-alkoxycarbonyl- or 1-aminocarbonyl-3-methyl-4-acylthio-1 H-pyrazol-5( 2H)-ones. The same adducts in chloroform with trifluoroacetic acid under reflux produce 1-alkoxycarbonyl- or 1-aminocarbonyl-3-methyl-4-acylthio- 5-alkoxypyrazoles. In acidic media with aqueous tetrahydrofuran the 1,4-adducts undergo hydrolysis affording 2-acylthio-3-oxobutanoate derivatives that exhibit enol-keto tautomerism. The 1-alkoxycarbonyl- and 1-aminocarbonyl-3-methyl-4-acylthio-1 H-pyrazol- 5(2 H)-one derivatives in methanol under reflux undergo solvolysis to give, after a few minutes, simple 3-methyl-4-acylthio- 1 H-pyrazol-5(2 H)-ones and, after a few' hours, 4,4′-dithiobis(3-methyl-1 H-pyrazol-5(2 H)-ones), while 1-alkoxycarbonyl- and 1-aminocarbonyl-3-methyl-4-acylthio-5-alkoxypyrazole derivatives are resistant to the same reaction conditions even over several days. X-Ray diffraction studies of some 1-alkoxycarbonyl-and 1-aminocarbonyl-3-methyl-4-acylthio- 1 H-pyrazol-5(2 H)-ones have been performed in order to determine the structure of the pyrazole ring in all such compounds, as well as the conformational situation in the solid slate of the substituents on N-1. These investigations confirm the reconsideration of the structure assignments frequently recurring in the literature for some 5- and 3- hydroxypyrazoles.