Proper characterization of solute segregation during the solidification of alloys using partition coefficients (k) is essential to understand and model solidification paths and associated microstructural evolution. Traditional methods to determine k assume a constant value over the course of solidification, which leads to improper descriptions of segregation for elements with variable partitioning behavior. In this study, a novel method to determine partition coefficients as a function of fraction solid using electron probe microanalysis point grid measurements is described. The proposed method, termed the mass balance analysis (MBA), was applied to a cast 90 wt pct Co-10 wt pct Cu alloy with well-documented segregation behavior. The resultant partition coefficients indicated that kCu decreased as a function of fraction solid, while kCo significantly increased in the terminal stage of solidification. The measured k values showed excellent agreement with those taken from the solidus and liquidus lines of Co-Cu binary phase diagram generated using Thermo-Calc. The MBA was also applied to a multicomponent austenitic commercial alloy. It showed a decrease in the partition coefficients for both Cr and Nb as a function of fraction solid and demonstrated the applicability of the MBA method to highly alloyed multicomponent systems.