Fractional crystallization (FC)-Modeler is a user-friendly, interactive Microsoft Excel spreadsheet program that models Rayleigh fractionation vectors and produces high-quality graphic output in the form of log-log bivariate diagrams. The design of the program provides the user with an opportunity to undertake petrologic modeling. Due to its interactive nature, this program updates the output simultaneously, when any parameter is changed, allowing the user to simulate the FC process. FC-Modeler also permits plotting geochemical data beside the FC vectors and hence it can be used as a powerful tool to constrain the nature and amount of the FC in a closed magmatic system. (C) 2002 Elsevier Science Ltd. All rights reserved.