Automatic crystal orientation mapping (ACOM in the SEM and orientation determination in the TEM are used for a precise and detailed characterization of the microstructure of a multiphase TRIP steel. Besides austenite, bainite plays an important role for the functioning of the TRIP effect. Therefore, particular emphasis is placed on the difficult discrimination of bainite and ferrite. The existence of massive bainite is concluded from the interpretation of characteristic orientation gradients in the ferrite and a Kurdjumov-Sachs orientation relationship of bainite and austenite. The bainite formation is explained by a reversion of the gamma-alpha phase boundary movement during austenization and subsequent tempering at bainite formation temperature.