In the presence of mild base such as triethylamine, the two 4-alkylidene-2-phenyl-2-oxazolin-5-ones, 1 and 8, rapidly absorb oxygen at room temperature with the loss of carbon dioxide and the formation of the imides 4 and 10. The imide probably arises by decomposition of a hydroperoxide resulting from the reaction of oxygen and the anion of the oxazolinone. A hydroperoxide intermediate will also account for the origin of the acidic by-products 6a and 11a. The 2-methyl-2-oxazolin-5-one 14 did not react with oxygen at room temperature; at 60° some reaction occurred but no imide was formed.