Due to the inherent brittleness of γ titanium aluminide alloys part of the alloy and processing development aims at fine and homogeneous microstructures. Pronounced microstructural refinement can be achieved by the addition of B, as it is known since about two decades. Recently it has been shown that this effect can be attributed to heterogeneous nucleation of the α phase on borides during the β → α transformation, which however is only observed for slow cooling rates. In order to understand the microstructural refinement the phase transformation kinetics was analyzed by in situ high-energy XRD on cooling the material from the high-temperature β phase field. In the experiments an inductively heated dilatometer was used for heating and cooling specimens of the alloys Ti-43Al-5Nb and Ti-43Al-5Nb-0.2B (at.%). The specimens were heated to a temperature above 1430 °C, held for 1 min and then cooled to room temperature with several cooling rates from 1 K/s up to 30 K/s. The phase fractions were determined by HEXRD and in addition from the length signal measured by the dilatometer. The obtained transformation curves were quite similar except some characteristic differences. From the evolution of the transformed volume fraction the start temperature of the β → α transformation was determined for different cooling rates. For all cooling rates the transformation starts at higher temperature in the B containing alloy. Further, the experiments indicate a change in the transformation mechanism between 5 K/s and 10 K/s the origin of which will be discussed in the contribution.