Measurements of the extraordinary/ordinary molar refractivity and thermal expansion coefficient as a function of temperature are presented for the lyotropic mixture of potassium laurate, decanol and water. The measurements cover the range of the calamitic nematic phase (N-C) and the vicinity of the reentrant isotropic (I-RE) -N-C- isotropic (I) phase transitions. Thermal expansion coefficient (beta) was obtained from density data while that the refractivity parameters were determined through of the Vuks's equation together with refractive indices and density values. These parameters are discussed in terms of structural changes in the micellar configuration at the phase transitions.