Many phenolic compounds are known to enhance the chemiluminescence associated with the horseradish peroxidase-catalyzed oxidation of luminol, but the mechanism of enhancement is still unproved. Using stopped-flow spectrophotometry, we have found that a series of luminescence enhancers react rapidly with the peroxidase reactive intermediates (compound I and compound II) supporting the hypothesis that the enhancement is due to the acceleration of the enzyme turnover. In addition, pulse radiolysis experiments have shown that the enhancers' phenoxyl radicals oxidize luminol, consistent with a redox mediator role for the enhancers. The latter reaction was found to be reversible, showing that enhancers of low reduction potential, which are efficient in accelerating the enzyme turnover, are also scavengers of luminol radicals and therefore luminescence quenchers. Using these data, a simple model is proposed which correctly predicts that the efficiency of a phenolic compound as luminescence enhancer depends on the reduction potential of the respective phenoxyl radical according to a bell-shaped function with a maximum at ~0.8 V.