The ionic strength dependence of the bimolecular electron transfer (ET) reaction between Clostridium pasteurianum rubredoxin(III) and photochemically generated [Ru(bpy)(3)](+) has been investigated. The reaction is fast and close to the diffusion-controlled limit. Dynamic ionic strength effects on all kinetics parameters (work terms, driving force, and reorganization free energy) have been incorporated in the data analysis; the effects on the intermolecular work terms are large and dominate in the driving force region close to the activationless limit. The variation in ET rate constant over the 0.005-2.0 M ionic strength range is quantitatively consistent with the high negative charge (-9e where e is the unit electric charge) and size (crystallographic radius approximate to 12 Angstrom) of rubredoxin(III). If low ionic strength (0.005-0.01 M) data are omitted, the variation in rates suggests a somewhat smaller charge on the protein (-4e to -5e).