Models have been proposed to describe the rheology of immiscible polymer blends in relation to their morphologies, either for dilute droplet emulsions or for concentrated blends with complex interfaces. The morphologies obtained in reactively compatibilized blends are the result of a complex interplay between instabilities due to chemical reaction at interfaces and drop breakup and coalescence processes. We have studied reactively compatibilized Polyamide6/High Density Polyethylene (PA6/HDPE) blends with widely different viscosity ratios, obtained by changing the length of PA chains. The composition at phase inversion is shifted to high concentration of the most viscous component, as may be expected from simple rheological considerations. The relationships between the morphologies and the rheological behavior are discussed. The objective of this work is to relate the rheological behavior to the morphologies. Specifically, we show that phase inversion can be reasonably predicted, based on the rheological behavior of blends in the linear regime. This result could be of significant benefit for applications, since rheological studies are often easier to perform in the applicative context than electron microscopy observations, which are needed to characterize the morphologies at sub-micrometric scales.