AFC-Modeler is a user-friendly, interactive Microsoft Excel workbook program which is specifically designed to model up to ten theoretical AFC curves (corresponding to ten different r values; r being the ratio of mass assimilation rate to fractional crystallization rate) for a set of variables that can be interactively and precisely modified by the user (i.e. D, initial element concentration and isotope ratio in magma and element concentration and isotope ratio in wallrock). Being able to model ten curves for ten different 'r' values is an important feature of the program, because results of many studies indicate that the r values for a magmatic suite usually vary widely instead of following a single curve. Particular care has been taken in the design of the program in order to turn rather complex modelling into a simple and error-free procedure by utilizing a specifically designed graphical user interface, consisting of combo boxes placed around a scatter chart which continuously displays up-to-date results of a petrological model. It enables the user of the program to plot elements, ratios of elements and radiogenic (i.e. Sr, Nd and Pb) and stable (delta O-18) isotopic ratios against each other. Up to eight data series can be entered into the program and stored on eight separate spreadsheets and then plotted along with the modelled theoretical curves. This enables the user to compare the modelled curves and the trends shown by his/her magma series and hence enables him/her to make interpretations and estimations about the degree of crustal assimilation in magma genesis. The program also has two separate and user-modifiable sheets for storing D values of the elements for basic, intermediate and acid magmas, and storing up to 100 magmatic/crustal end-member compositions, which can be utilized in the modelling. By virtue of these features, the AFC-Modeler program can be used as a useful tool for both research and educational purposes.