The coefficient of electronic polarizability (phi(i)) is determined by means of the density and extraordinary/ordinary refractive indexes measurements near the reentrant isotropic (I-RE) - calamitic nematic (N-C) and N-C - isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and water. These parameters were related to the extraordinary/ordinary refractive indexes via the temperature derivative of the Vuks's equation. The results showed that near the I-RE-N-C and N-C-I phase transitions the measured value of phi(i) was found to be about two order of magnitude smaller than that obtained for nematic MBBA system. Sign inversion of this coefficient near these phase transitions is also discussed.