The nature of the ions that are translocated by Escherichia coil and Paracoccus denitrificans complexes I was investigated. We observed that E. coil complex I was capable of proton translocation in the same direction to the established Delta Psi, showing that in the tested conditions, the coupling ion is the H+ Furthermore, Na+ transport to the opposite direction was also observed, and, although Na+ was not necessary for the catalytic or proton transport activities, its presence increased the latter. We also observed H+ translocation by P. denitrificans complex I, but in this case, H+ transport was not influenced by Na+ and also Na+ transport was not observed. We concluded that E. coli complex I has two energy coupling sites (one Na+ independent and the other Na+ dependent), as previously observed for Rhodothermus marinus complex I. whereas the coupling mechanism of P. denitrificans enzyme is completely Na+ independent. This work thus shows that complex I energy transduction by proton pumping and Na+/H+ antiporting is not exclusive of the R. marinus enzyme. Nevertheless, the Na+/H+ antiport activity seems not to be a general property of complex I, which may be correlated with the metabolic characteristics of the organisms.