This work focuses on quantitatively characterizing and classifying the modification of Al7Si and Al7Si0.3Mg casting alloy microstructures. Different levels of modification were induced through the addition of Sr and varying cooling rates during solidification. Firstly, a round robin test (RRT) was developed in which representative micrographs were subjectively evaluated by a group of experts in the area of aluminum casting alloys, from thirteen different laboratories. Secondly, a quantitative evaluation of the micrographs was conducted using an image analysis software. Lastly, a classification model was generated with the help of a data mining software, combining the quantitative data and the experts' evaluations. Different classes of microstructures were quantitatively defined, allowing objective assessment of the microstructures. Additionally, a quantitative degree of modification (M), which depends on the microstructure homogeneity, was developed to evaluate the modified microstructures. The result of this work lays the foundation for a forthcoming study of the quantitative correlation between process settings, microstructures, and casting properties.