A mechanistic study into the copper(i)-catalysed sulfonylative Suzuki–Miyaura reaction, incorporating sulfur dioxide, is described. Utilising spectroscopic and computational techniques, an exploration into the individual components of the competing catalytic cycles is delineated, including identification of the resting state catalyst, transmetalation of arylboronic acid onto copper(i), the sulfur dioxide insertion process, and the oxidative addition of aryl halide to CuI. Studies also investigated prominent side-reactions which were uncovered, including a competing copper(ii)-catalysed mechanism. This led to an additional proposed and connected CuI/CuII/CuIIIcatalytic cycle to account for by-product formation.