Porphyry tin and tin greisen deposits are the most important primary source of tin. They are typically linked to S-type granites that also show significant enrichments of rare metals, such as gallium and indium. While there have been a number of studies on the behavior of tin in magmatic-hydrothermal systems, experimental data for the highly reducing conditions of S-type granites are missing. We propose to fill this gap by acquiring cassiterite (SnO2) solubility data in hydrothermal fluids as well as in-situ Raman data on Sn speciation. We will use these data to develop models for the enrichment of tin in granitic systems that can be used to identify the mineralization potential of intrusions. In addition, we will carry out some first exploratory experiments on the magmatic-hydrothermal geochemistry of gallium and indium in order to better understand the factors leading to the enrichment of these elements. Since there is an increasing demand for gallium and indium in semiconductor industry, new sources for these rare metals may have to be developed in the future.