The oximation of acetyl(hydroxyimino)acetone (1) with hydroxylamine affords a mixture of oxime derivatives. The products obtained by the action of an equimolar amount of hydroxylamine on (1) are the two 3,5-dimethyl-5-hydroxy-4-hydroxyimino-2-isoxazoline isomers (3a and b). The main products obtained by the action of an excess of hydroxylamine on (1) are two pentane-2,3,4-trione trioxime isomers (4a and b) and a little amount of 4-acetyl-3-methylfuroxan oxime (5). N.m.r. and chemical data of all these derivatives are discussed in order to determine both structures and configuration. X-Ray analysis of (3a), (4a), and (5) are also reported.