The cytochrome bc 1 complex (commonly called Complex III) is the central enzyme of respiratory and photosynthetic electron transfer chains. X-ray structures have revealed the bc 1 complex to be a dimer, and show that the distance between low potential (b L ) and high potential (b H ) hemes, is similar to the distance between low potential hemes in different monomers. This suggests that electron transfer between monomers should occur at the level of the b L hemes. Here, we show that although the rate constant for b L →b L electron transfer is substantial, it is slow compared to the forward rate from b L to b H , and the intermonomer transfer only occurs after equilibration within the first monomer. The effective rate of intermonomer transfer is about 2-orders of magnitude slower than the direct intermonomer electron transfer.