The blend polyamide 6 (PA-6)/poly(methyl methacrylate) (PMMA) can be compatibilised by means of the reactive copolymer styrene-maleic anhydride with 20 wt% maleic anhydride (SMA20). The region of phase co-continuity has been analysed for the non-compatibilised and the compatibilised blends with scanning electron microscopy (SEM). The blend composition range of phase co-continuity was shifted to a lower PA-6 content when compatibiliser (SMA20) was added and the co-continuous structures were observed over a much narrower composition range. The effect of the molecular weight of PA-6 on the composition range of phase co-continuity was also analysed; it was found to be shifted to lower PA-6 content with decreasing molecular weight of PA-6. This shift could be interpreted by measuring the melt-viscosity of the different polymers using capillary rheometry. The analysis of the dynamic mechanical properties of the blends confirmed the results obtained with SEM; a clear change in storage modulus (log E＇) and damping (tan 6) was observed in the phase inversion region. Finally, the crystallisation behaviour of PA-6 in the blends was investigated during cooling from the melt with differential scanning calorimetry. Fractionated crystallisation was observed in the blends where PA-6 forms the dispersed phase.