The definitive version is available at: http://www.rsc.org/publishing/journals/PP/The detergent effect on Cytochrome b559 from spinach photosystem II was studied by electron paramagnetic resonance (EPR) spectroscopy in D1–D2–Cyt b559 complex preparations. Various n-dodecyl--D-maltoside concentrations from 0 to 0.2%(w/v) were used to stabilise the D1–D2–Cyt b559 complexes. Low spin heme EPR spectra were obtained but the gz feature positions changed depending on the detergent conditions. Redox potentiometric titrations showed a unique redox potential cytochrome b559 form (Em=+123–150 mV) in all the D1–D2–Cyt b559 complex preparations indicating that detergent does not affect this property of the protein in those conditions. A similar effect on Cytochrome b559 EPR spectrum was observed in more intact photosystem II preparations independently of their aggregation state. This finding indicates that changes due to detergent could be a common phenomenon in photosystem II complexes. Results are discussed in terms of the environment each detergent provides to the protein.I. G.-R. was recipient of a fellowship from the Ministry of Education and Culture of Spain. This work was supported by the Dirección General de Investigación Científica y Técnica (Grants PB98-1632 to R.P. and PB97-1135 to J.M.O.) and by the Diputación General de Aragón (Projects P17/98 to P.J.A. and P111/2001 to J.I.M.).Peer reviewed