Geochemical Data Toolkit (GCDkit) is a program for handling and recalculation of geochemical data from igneous and metamorphic rocks. It is built using the Windows version of R, which provides a flexible and comprehensive language and environment for data analysis and graphics. GCDkit was designed to eliminate routine and tedious operations involving large collections of whole-rock data and, at the same time, provide access to the wealth of statistical functions built into R. Data management tools include import and export of data files in a number of formats, data editing, searching, grouping and generation of subsets. Included are a variety of calculation and normative schemes, for instance CIPW and Mesonorm, as are the common geochemical graphs (e.g. binary and ternary graphs, Harker plots, spider plots, and several dozens of classification and geotectonic discrimination diagrams). The graphical output is publication ready but can be further retouched if required. The system can be further expanded by means of plug-in modules that provide specialist applications. GCDkit is available as Free Software under the terms of the Free Software Foundation's GNU General Public License and can be downloaded from http://www.gla.ac.uk/gcdkit. The product is actively maintained and updated to provide additional functionality; Unix/Linux and Mac OS versions are being developed.