The phase morphology developed in immiscible polypropylene (PP)/polycyclohexylmethacrylate (PCHMA) blends has been studied using an in situ reactively generated polystyrene-graft-polypropylene compatibilizer from maleic anhydride grafted polypropylene (MA-g-PP) and amine end-capped polystyrene (PS-NH2) reactive precursors during melt-blending. The imidation reaction responsible for the formation of the compatibilizer is similar to the reaction occurring in polyamide/MA-PP (MA-EPR or MA-EPDM) blends which are industrially important. In the present blend PP/PCHMA/(PP-MA-PS-NH2), no undesired reaction occurs between the maleic anhydride groups and the backbone of the PCHMA chain, as is usually the case with polyamide homopolymer. This type of reaction, although considered non significant, has consequences on the phase morphology development as it affects the viscosity of the polyamide matrix when chain scission takes place. PP/PCHMA blends covering the whole range of compositions were prepared. The composition window at which the blends exhibit a droplet-in-matrix phase morphology and that where the two phases are co-continuous were determined using a selective phase extraction in combination with scanning electron microscopy. The generation in situ of the PP-g-PS compatibilizer substantially changed the state of the phase morphology developed. In the blends having a droplet-in-matrix type of morphology, the particle sizes were significantly reduced (by a factor of more than 10). Two types of MA-g-PP reactive copolymers differing in maleic anhydride content (1 and 8 wt%) have been separately employed with the same grade of PS-NH2. Emphasis was put on a detailed investigation of the behaviour and structural stability of the blends exhibiting a co-continuous phase morphology when the compatibilizer is generated. Significant differences were found in relation to the maleic anhydride content of the MA-PP reactive compatibilizer precursor. (C) 2004 Published by Elsevier Ltd.