Alterations in fluidity of thylakoid membranes isolated from spinach chloroplasts in response to sodium bisulfite (NaHSO 3 ), hydrogen peroxide (H 2 O 2 ), sodium dodecyl sulfate (SDS), bovine serum albumin (BSA), and free linoleic acid (LA) were investigated by means of a fluorescence polarization study with 1,6-diphenyl-1,3,5-hexatriene as the fluorescence probe. A decrease in fluidity and an increase in microviscosity of membrane were caused by NaHSO 3 and H 2 O 2 treatment. In contrast, SDS and BSA were found to increase thylakoid membranes fluidity and decrease microviscosity, in which the corresponding correlation coefficients were -0.9995 to -0.9516 (SDS) and -0.9359 (BSA), respectively. No changes in thylakoid membranes fluidity induced by free LA were found until its concentration above 5 mM where the polarization value (P value) declined (increased fluidity). The results suggest that the changes in thylakoids membrane fluidity might depend on the characteristics, mechanism and extent of the interactions between membrane components and compounds added.