Abstract Quinoxaline 1,4-dioxide (XIIIa) with acetic anhydride gave 1-acetoxy-2(1H)-quinoxalinone (XIVa) which was prone to facile hydrolysis to yield 1-hydroxy-2(1H)-quinoxalinone (XVa). Both XIVa and XVa were isolated from the reaction mixture. On prolonged heating with acetic anhydride, XIIIa, XIVa, and XVa were converted slowly to the same end product, 2,3(1H,4H)-quinoxalinedione (XXa). 6-Ethoxy- (XIIIb), 6-methoxy- (XIIIc), and 6-methylquinoxaline 1,4-dioxide (XIIId) behaved similarly, except that the attack of the reagent took place exclusively on N-oxide para to the electron-donating substituents, and none of the other expected isomeric compounds XVIIb–d were isolated. Whereas 6-chloroquinoxaline 1,4-dioxide (XIIIe) bearing an electron-attracting chloro substituent on the benzene ring gave exclusively the other isomers XVIIe, XVIIIe, and XXe. A mechanism for this novel rearrangement is proposed and discussed.