This work deals with the study of the phase inversion phenomenon in blends of poly(m-xylylene adipamide) and maleic anhydride functionalized polypropylene (PAmXD,6/PP-g-MA blends) processed in a Brabender plastograph at 265 +/- 5 degrees C and 45 rpm. The viscosity of the components has been modelized by the Brabender torque and the phase inversion composition was determined by means of a solvent dispersion technique (SDT). The compatibilization, i.e., the amount of copolymer in the blend, does not modify the phase inversion location. The phase inversion composition is determined early during the process and is weakly or even not at all affected by further modification of viscosity ratio of the components versus mixing time. This work demonstrates that the only key parameter of the phase inversion composition is the viscosity ratio of the components at the first stage of the mixing process. An empirical equation linking volume fraction ratio for the phase inversion composition and the square root of the viscosity ratio of the components is proposed.