The dediazoniative arylation of various aromatic hydrocarbons (Ar’H) with diazonium salts (ArN2+) in acetonitrile can be readily effected to biaryls (Ar-Ar’) in high yields, simply by the addition of small (catalytic) amounts of sodium iodide. [In the absence of Ar’H, the competitive iodination to ArI is nearly quantitative.] Iodide catalysis of biaryl formation is efficiently mediated by aryl radicals (Ar-.) that participate in an efficient homolytic chain process in which ArN2+ acts as a 1-electron oxidant. The complex kinetics of such an electron-transfer chain or ETC process (Scheme 1) is quantitatively verified by computer simulation of the Ar’H-dependent (a) competition between arylation vs iodination and (b) catalytic efficiency of iodide, using the GEAR algorithms. ETC catalysis also pertains to the alternative photochemical procedure for arylation (in the absence of iodide), in which the deliberate irradiation of the charge-transfer band of the precursor complex [ArN2+, Ar’H] initiates the same homolytic chain arylation. The latter underscores the mechanistic generality of the ETC formulation for various types of catalytic dediazoniations of aromatic diazonium salts.