Effect of anions on thylakoid membrane fluidity as measured by rotational correlation time, order parameter (by ESR spectroscopy) and fluorescence polarization was studied. The data in the present study offer proof that Mg(2+)-induced rigidity of thylakoid membranes could be reversed by anions like bicarbonate and chloride. Anions were found to play a role in structural reorganization of thylakoid membranes.