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The deformation substructure of TiAl (Llo type ordered lattice) tested in compression, and the factors determining it were investigated. Two types of dislocations take part in the plas-tic deformation, namely a/2 [110] and a/2 [Oil]. The latter type will disorder the Llo super-lattice and therefore would be expected to move in pairs as superdislocations. Some obser-vations are essentially in agreement with the predictions, however the large proportion and morphology of a/2 [O1l] dislocations observed was unexpected. Twins of the [112] (111) type play an important role in the deformation of the alloy, and the early stages of their formation have been recorded. Finally, equi-Schmid factor lines have been constructed in an attempt to evaluate the importance of the sense of the applied stress on the deformation capability of the alloy.