Thermochemical processing (TCP) with hydrogen, including hydrogenation, intermediate heat treatment and dehydrogenation, of super-alpha2, a type of Ti3Al-based titanium aluminide alloy, was carried out in order to examine the microstructural variations in the alloy. It is shown that a number of microstructural changes, such as the amount, morphology and distribution of different phases in the alloy, can be brought about by TCP. Acicular or Widmanstatten alpha2 was found to form in the retained B2 matrix under TCP with an intermediate heat treatment of furnace cooling from high temperature, instead of mainly equiaxed alpha2 under the normal treatment. The precipitation in the microstructures for alloys undergoing TCP with the intermediate beta-solution heat treatment followed by ageing while dehydrogenating was found to be more regular in morphology and homogeneous in distribution. Both the amount of primary alpha2 and the size of the primary beta grains in the microstructure could be controlled by suitable TCP. Some of the reasons leading to the microstructural variations caused by TCP are discussed.