The compatibilisation of immiscible blends of polyamide 6 (PA-6) and poly(methyl-methacrylate) (PMMA) using a reactive copolymer styrene-maleic anhydride (SMA) has been investigated. In a first series of experiments, SMA20 (20 wt% MA) was used as the reactive copolymer, and this compatibilisation method turned out to be quite efficient. Five to 6 wt% SMA20 copolymer was needed to obtain the minimum particle size for the dispersed PMMA-phase in the blend PA-6/PMMA (75/25). From melt-blending experiments during which the phase morphology was analyzed as a function of the extrusion time, it was concluded that the diffusion of SMA towards the PA-6/PMMA interface is the rate limiting step for the formation of the graft copolymer PA-6-g-SMA and as a consequence for the reactive compatibilisation process. By varying the percentage dispersed phase of the blends, it was possible to examine the role of coalescence with respect to the size of the dispersed particles. It was concluded that the reduction of coalescence in the compatibilised blends is the main reason for the dispersed phase particle size reduction. The miscibility between PMMA and SMA copolymers with a varying MA content ranging from 10 up to 35 wt% MA offers a unique opportunity to study the effect of the degree of functionality of the reactive copolymer on the compatibilisation process. The use of SMA copolymers with a different MA content revealed that the functionality of SMA is a very critical parameter with respect to its compatibilising efficiency in the blends because it influences the miscibility of SMA with PMMA. Finally, experiments with low-molecular weight PA-6 revealed that the added SMA compatibiliser is more efficiently used in this case.