The transient electron transfer (ET) interactions between cytochrome c 1 of the bc 1-complex from Paracoccus denitrificans and its physiological redox partners cytochrome c 552 and cytochrome c 550 have been characterized functionally by stopped-flow spectroscopy. Two different soluble fragments of cytochrome c 1 were generated and used together with a soluble cytochrome c 552 module as a model system for interprotein ET reactions. Both c 1 fragments lack the membrane anchor; the c 1 core fragment ( c 1CF) consists of only the hydrophilic heme-carrying domain, whereas the c 1 acidic fragment ( c 1AF) additionally contains the acidic domain unique to P. denitrificans. In order to determine the ionic strength dependencies of the ET rate constants, an optimized stopped-flow protocol was developed to overcome problems of spectral overlap, heme autoxidation and the prevalent non-pseudo first order conditions. Cytochrome c 1 reveals fast bimolecular rate constants (10 7 to 10 8 M − 1 s − 1 ) for the ET reaction with its physiological substrates c 552 and c 550, thus approaching the limit of a diffusion-controlled process, with 2 to 3 effective charges of opposite sign contributing to these interactions. No direct involvement of the N-terminal acidic c 1-domain in electrostatically attracting its substrates could be detected. However, a slight preference for cytochrome c 550 over c 552 reacting with cyochrome c 1 was found and attributed to the different functions of both cytochromes in the respiratory chain of P. denitrificans.