The EPR spectral parameters of aa3 oxidase and cyt c552 from Paracoccus denitrificans were studied in purified oxidase and enriched cyt c552. The orientation of the g-tensors of hemes a and c552 were determined on partially ordered membranes, enriched cyt c552 and a c552:aa3 subcomplex. The known correlation of g-tensor to molecular axes in histidine/methionine ligated hemes permits us to position cyt c552 with respect to the parent membrane. Taken together with previous data on the interaction surface between aa3 oxidase and cyt c552, these results allow us to arrive at a single conformation for the c552:aa3 electron transfer complex. Cop. 2006 Federation of European Biochemical Societies.