Abstract In the last decades the importance of metals and metallic alloys underlies an increasing importance, and with it the requirements on properties and quality of alloys. Important commercial alloys are for example steel, aluminum, copper, tin, and zinc alloys. Due to the fact that they show peritectic reactions in the phase diagram, it is of great importance to improve the understanding of a peritectic reaction and related morphologies leading to improved material properties of high quality. In the last century great efforts were made to gain deeper understanding of the microstructure formation in peritectic alloys during solidification, especially in cases where both phases solidify as a planar front. The formation of a microstructure from the melt is influenced by convection in front of the solid/liquid interface, a consequence of the existing gravity on earth. Without gravity the natural convection does not operate, such as in the orbiting International Space Station (ISS). Therefore, the European Space Agency (ESA) supports investigations on peritectic solidification morphologies within the frame of the project “Metastable Solidification of Composites” (METCOMP). To estimate the influence of natural convection on solidification morphologies the investigations within this project were divided into ground experiments, under normal gravity, and space experiments, under micro gravity. Due to a delay of the construction of the Bridgman-furnace for in-situ observation of direct solidification (DIRSOL) in space by ESA, the experiments in space had to be postponed to 2014. Thus the influence of the natural convection on the solidification morphologies remains to be elucidated in detail. Investigations on peritectic metallic systems show a wide range of possible microstructures. Bands, tree-like microstructures, islands, and coupled growth were detected at a growth rate where both phases can solidify in form of a planar front. To improve the understanding of appearing morphologies during solidification transparent model systems for in-situ observation are an attractive option. Such systems offer the advantage that both, the morphology and the dynamics of solidification can be investigated by using optical diagnostic means. The organic phase diagram TRIS - NPG was selected for this study because temperature and concentration of the peritectic point are suitable for direct observation in a micro Bridgman-furnace setup. The organic compound TRIS is used for in-situ observation for the first time. Therefore, additional investigations to complete the physical properties of TRIS and the alloys of TRIS - NPG had to be performed. Whereby, thermal instability of the organic compound TRIS was detected that constrains the processing window for in-situ observations. The investigations on the organic phase diagram TRIS – NPG indicated a wide range of microstructures, whereby, well known structures as well as ones were found close to the limit of constitutional undercooling at the peritectic region. Oscillating behavior was found close to the peritectic concentration at pulling rates above the limit of constitutional undercooling. Here, both phases grow in a competitive manner in a way that oscillating solidification occurs. This kind of solidification was observed for the first time and no literature has been found up to now that describes this behavior. At a solidification rate close and below the limit of constitutional undercooling only a planar solidification front was found. In a few cases the growth of isothermal peritectic coupled growth (PCG) or banded growth which lead to isothermal PCG was observed. Evidence for this form of transformation were found in experiments with metals and supported with numerical simulation. Here, it was the first time that the transformation from banded to PCG is reported by direct observation.