3-Hydrazinylquinoxalin-2(1H)-one reacts with ethyl 2-hydroxyimino-3-oxobutanoate or 1-Ar-2-(hydroxyimino)-butane-1,3-diones (Ar = Ph, 4-Me-C6H4, 4-F-C6H4, 4-Cl-C6H4, 4-Br-C6H4, naphthalen-1-yl) in acetic acid to form a mixture of the E- and Z-hydrazones in excellent yields. The reaction of 3-hydrazinylquinoxalin-2(1H)-one with 1-(pyridin-4-yl)-2-(hydroxyimino)butane-1,3-dione gave 3-[5-hydroxy-4-(hydroxyimino)-3-methyl-5-(pyridin-4-yl)-4,5-dihydro-1H-pyrazol-1-yl]quinoxalin-2(1H)-one. The reaction with 2-hydroxyimino-3-oxobutanal in ethanol leads to 3-{2-[2-(hydroxyimino)-3-oxobutylidene]hydrazinyl}quinoxalin-2(1H)-one. The structures of the synthesized compounds were determined by UV–Vis, IR, and 1H and 13C NMR spectroscopy, chromatography–mass spectrometry, and elemental analysis. Proton signals were assigned using 2D homo- (1H–1H COSY) and heteronuclear (1H–13C HSQC, 1H–13C HMBC, 1H–15N HMBC) experiments. [ABSTRACT FROM AUTHOR]