High molecular weight, linear aromatic poly(ether ketone ketone)s with different iso-/tere-isomer repeating unit ratio (100/0, 50/50, 0/100) have been studied by H-1 and C-13 n.m.r. spectroscopy. The polymers were obtained in particle form by precipitation electrophilic Friedel-Crafts acylation condensation of iso- and terephthaloyl chlorides with diphenyl ether, 1,4- and 1,3-bis(4-phenoxybenzoyl)benzenes. Conventional H-1 and C-13 n.m.r. spectroscopy shows para-substitution in the diphenyl ether fragments of the main chain. Analysis of the expanded H-1 n.m.r. spectra for the ring proton resonances reveals defect meta- and orthostructures. The amount and isomer ratio of these defect structures depend on the polymer structure and preparation path. The low field of H-1 n.m.r. spectra of polyketones based on ’small monomers’ (diphenyl ether and iso- and terephthaloyl chlorides) were found to show additional minor signals corresponding to a different type of defect structure. A possible mechanism of formation of these defect structures is discussed.