The thermal stability of purple membranes is studied by electric light scattering. Information on the polarization mechanisms is obtained. Incomplete recovery of the initial electric state (i.e., of permanent dipole moment, p(perpendicular to), and electric polarizability, gamma(fast), (parallel to)) after the membranes are heated at temperatures above 60 degreesC is revealed. Additional slow polarizability components, gamma(slow),(perpendicular to) and gamma(slow,parallel to) , relaxing at different characteristic frequencies than the fast longitudinal polarizability gamma(fast),(parallel to) appear in the temperature range where the order-disorder transition takes place. The slow polarizability gammaslow,(perpendicular to) is probably connected with counterion displacement in the electrical double layer perpendicular to the disk plane. The results are important for understanding the polarization mechanisms and the origin of slow orienting moments. (C) 2003 Elsevier Science (USA). All rights reserved.