The reaction of 5-ring ketene-O,N-acetals with peroxides was investigated. It was shown that benzoyl peroxide adds to monomers 5a and 5b by ring opening, giving the corresponding linear diester amides 6a and 6b, respectively. The ketene-O,N-acetal 5c adds benzoyl peroxide, without ring opening, by addition to the exomethylene group, giving the cyclic-O,N-acetal diester 6c. With phthaloyl peroxide cyclic esteramides 7 and oligomeric products are formed. The chemical structures of the addition products were confirmed by NMR spectra and elemental analysis.