Isothermally coupled peritectic solidification for a hyper-peritectic alloy under pure diffusive conditions is presented. For this purpose, directional solidification experiments were performed aboard the International Space Station using a model system for peritectic layer solidification patterns. At constant temperature gradient of 3.0 K/mm and for pulling velocities ranging from 0.12 μm/s to 0.09 μm/s, coupled peritectic growth was observed. At lower pulling velocities, contrary to expectations, only a planar solidification front of the pro-peritectic phase was detected. Two effects were noticed: a significant effect of the pro- peritectic interface on the capability of the peritectic phase to nucleate and, in the further course of the
experiments, a dynamic change of the coupled peritectic growth microstructure.