Directional transformation of a hypo-peritectic Fe–17.5 at.% Co alloy was studied. Two consecutive phase transformations—solidification (liquid to delta ferrite) and solid-state transformation (δ ferrite to γ austenite)—were observed and compared with theory. In all experiments, the solidification front was planar and in the steady-state, and therefore produced a homogeneous parent phase for the following δ–γ transformation. Depending on the growth conditions, γ transformed from δ as cells or as a plane front. The cell tip radius decreased with growth rate from V = 1–5 μm s−1. At higher velocities, between 7 and 10 μm s−1, the δ/γ interface morphology became planar. In order to explain this morphological transition, volume diffusion-controlled plane front growth and dendrite growth theory was applied. Good agreement was obtained between theory and experiments. It is concluded that plane front stabilization with increasing velocity is due to absolute stability, with a concentration spike at the transformation front. In the steady-state, this leads to composition invariance, typical for massive transformation. Computed interface velocities for quenching in heat treatment, which can be as high as several centimeters per second, show that, in certain cases, the controlling mechanism of massive transformation is steady-state plane front growth with a narrow concentration spike and not complete solute trapping.