The properties of steel vary strongly depending on the distribution of different phases present in its microstructure. Thus, it is of great importance to understand how the microstructures of steels are affected by the chemical composition and the heat treatment. The processes occurring during diffusive phase transformations are not well understood in some cases. Thus, efforts are made to simulate the kinetics of the austenite to ferrite phase transformation by means of physically based models (sharp-interface approximation and finite interface mobility). In such models the temperature regime, the prior austenite grain size, the ferrite grain size and the initial composition of the steel are the known quantities. In this thesis the austenite-to-ferrite transformation kinetics of an Fe-Cr-Ni alloy has been investigated experimentally. Each sample was heat treated in an identical way. The samples were heated up, austenised at a certain temperature and afterwards cooled down to room temperature. The cooling process included an isothermal segment so that a part of the austenite/ferrite transformation occurred at constant temperature. In the present thesis it is shown that the transformation kinetics proceeds in a reproducible manner, i. e. the transformation kinetics does not depend on the transformation cycle. A one-time homogeneous annealing and a low cooling rate are preconditions for this result. For the determination of the prior austenite grain size in the low carbon material, the method of thermal etching was developed further. By the application of organic glue as getter material it is possible to reduce the oxidation of the sample. In this way the prior austenite grain boundaries could be revealed clearly. In addition a Gibbs energy minimizer was developed for the system to be investigated. By means of this Gibbs energy minimizer a phase diagram can be calculated for the region with high iron content. Finally, these results were compared to phase diagrams which have been calculated by ThermoCalc.