Herein, in a microgravity campaign onboard the International Space Station, peritectic coupled growth (PCG) is studied by solidifying alloys of the peritectic transparent organic system TRIS–NPG under purely diffusive conditions. The experiment reveals that the formation of PCG begins with coagulated lateral bands comprising the peritectic phase at temperatures below the peritectic one. After reaching the growth front of the pro-peritectic phase, growth competition between both solid phases occurs such that the patches of the pro-peritectic phase, lamellae, and fibers/rods grow in the stated order, coupled with the peritectic phase. As the entire solidification process occurs during the initial transient stage, the diffusion coupling between the two solid phases is weak. With the gradual decrease in the interface temperature during the initial transient, the necessary growth conditions for the pro-peritectic phase
diminish. Additionally, different concentrations along the inclined solid/liquid interface favor the peritectic phase at the rear window and the pro-peritectic phase at the front window. The corresponding PCG lasts 19 hours while the recoil of the inclined interface continues to a temperature level that makes alterations of the ‘effective’ TRIS–NPG phase diagram necessary.