The cytochrome bc₁ complex is one of the enzymes that couples electron transfer to vectorial proton translocation in energy-transducing biomembranes. It is also one of the main sources of reactive oxygen species in the cell, and an early target for apoptosis-inducing molecules. Energy conservation in this complex is made possible by the obliged bifurcation of the electrons derived from quinol oxidation. Different models have been proposed to explain the mechanism which ensures this electron bifurcation. Some of these models consider that two quinone molecules occupy the quinol oxidation (Qo) site, while others propose the movement of a single quinone within the two domains that comprise this site. Crystallographic structures and mutational studies have shown the existence of movement of the soluble domain of the iron-sulfur protein (ISP), modifying previous models and generating new ones that assign to this movement a mechanistic role in electron bifurcation. In this review, the evidence supporting the different models of quinol oxidation in the bc₁ complex is critically analyzed.