Lyotropic nematic textures are investigated, using optical microscopy, near the reentrant isotropic (I-RE) - discotic nematic (N-D(-)) - biaxial nematic (N-B) - calamitic nematic (N-C(+)) - isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D2O. The N-B, intermediate phase between the two uniaxial nematic ones, is characterized by optical birefringence measurements. In this way, using a polarizing microscope and a colour CCD digital camera, the N-B - N-C(+) phase transition is identified as well as the domain of the N-B phase and N-D(-) - N-B transition point in accordance with optical birefringence data. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim