The aim of cracked naphtha selective hydrodesulfurization (SHDS) technologies, such as the SelectFining™ process licensed by UOP, is to reduce the sulfur content of the naphtha while simultaneously maintaining its octane value.  Both of these objectives are complicated by recombination, the term applied to the reaction of H₂S and olefins to form mercaptans.    A solid understanding of recombination reaction kinetics is critical in the design and operation of a selective hydrodesulfurization unit as the lowest achievable sulfur level is often dictated by the extent of recombination.  Furthermore, recombination contributes to the conversion of the valuable high octane olefins.  Recombination is a very rapid reaction which frequently, but not always, approaches equilibrium.  The extent of reaction is governed either by the equilibrium relationship or kinetics depending on the process conditions, feedstock composition, and reaction rates.  If the rate of desulfurization is slow relative to mercaptan formation, the extent of recombination will be driven by the mercaptan-olefin equilibrium.  Predicting the extent of recombination is a non-trivial challenge as it is impacted not only by the total olefin and H₂S content, but also the carbon number, double bond position, and branching of the olefins.  These issues will be discussed in the context of the results of a series of numerical simulations and pilot plant experiments.