PA6/PPO 70/30 blends were reactively compatibilized using carboxylated polystyrene (PS) and poly(styrene-block-4-methylstyrene) with various degrees of carboxylation. The high carboxylation of PS (up to about 50%) caused a decrease of dispersed PPO dimensions with a simultaneous deterioration of properties, especially of toughness and elongation. The best mechanical behavior was found for PS with 1% degree of carboxylation and for neat PS. On the other hand, degrees of carboxylation higher than 50% caused an increase in particle size. This was most significant for block copolymers, where a marked change in size and shape occurred, from spherical particles of about 1 mum in size to large, elongated particles about 50 mum long or a similar rough cocontinuous structure. The deteriorated mechanical behavior is tentatively explained by unsuitable properties of the reactively formed compatibilizer and thus of the interface. The enhanced rigidity of highly carboxylated poly(4-methylstyrene) chains (and its product of grafting with PA6), causing its decreased emulsification ability together with the expected rigid interface, which probably suppresses breakup of the PPO phase, may be responsible for the increase of the dispersed PPO dimensions found.