The interaction of electron transport inhibitors with the Rieske iron-sulfur center in chloroplast membranes and in a purified chloroplast cytochrome complex (the b6-f complex) has been studied by using electron paramagnetic resonance (EPR) spectroscopy. Several quinone inhibitors, all of which contain a halogen substituent and a bulky alkyl side chain, cause a shift in the EPR signal of the reduced Rieske iron-sulfur center from g = 1.90 to g = 1.94. This g-value shift occurs in untreated membranes as well as in the cytochrome complex, which contains the Rieske center and no other iron-sulfur centers. Other compounds known to inhibit electron transport in the region of the iron-sulfur center cause a smaller alteration in the EPR signal of the Rieske center and are able to interact with the quinone-binding sites as evidenced by their displacement of quinones from the Rieske center. One substrate, plastoquinone 9, was also able to displace quinones from the Rieske center while other, such as plastoquinone 1 and duroquinone, did not show this effect. These results are considered in relation to the mode of interaction of quinones with the Rieske center in the photosynthetic membrane.