We have earlier quantified the dependency of early nucleation events on the presence of diffusion fields in the direct vicinity of a newly forming cluster. We have shown that the Gibbs free energy barrier to the nucleation of an intermediate phase is highly influenced by atomic mobility and hence, cannot be neglected in nucleation theory. In the present investigation, we have applied these principles to a study of the industrially important and scientifically interesting δ-ferrite to austenite phase transition in the Fe-C system, which occurs at temperatures in excess of 1670K. By using high-temperature laser-scanning confocal microscopy, we have observed in-situ, in real time and at temperature that this reaction can occur by the progression of a planar growth front, by a cellular/dendritic morphology or by a massive-type of transformation. We will present our experimental results and propose an explanation for the occurrence of the massive-type of phase transformation.