Up to date very few organic substances have been reported that show a non-faceted/non-faceted (nf/nf) peritectic phase diagram in a temperature range suitable for direct observation in a micro Bridgman furnace setup. Sturz et al.  and Barrio et al.  studied the peritectic phase diagram for the organic model alloy TRIS (Tris(hydroxymenthyl)aminomethane) - NPG (Neopentylglycol). The phase diagram is based on thermal analysis by means of DSC measurements [1, 2] and evaluation of lattice parameters measured with x-ray diffractometry . In the current work we present investigations on the system TRIS – NPG that have been obtained by optical investigations of directional solidification in a micro Bridgman-furnace with various initial alloy concentrations and pulling rates in a fixed temperature gradient. The phase diagram [1, 2] was confirmed by direct comparison of DSC measurements and optical investigations. Furthermore we present in situ observations of solidification in the peritectic region. They show a solidification behavior that was clearly distinguishable from the solidification in hyper- and hypoperitectic regions of the phase diagram.