One of the outstanding problems and challenges in CO activation concerns the lack of a detailed mechanistic understanding of the roles of catalytic promoters used in a number of homogeneously catalysed carbonylation reactions. These problems, and attempts at their resolution, are highlighted with reference to (i) the varied range of promoters that have found use in composite catalysts for the direct synthesis of oxygenates such as ethylene glycol and ethanol from CO/H2, (ii) the promotional effects of N-bases in the catalytic methoxycarbonylation of alkenes to esters, and (iii) some preliminary ¹³C NMR spectroscopic evidence which is enabling the definition of a dual role of Ru-promoters as iodide abstraction agents in the Ir-catalysed carbonylation of methanol to ethanoic (acetic) acid. The detection, and characterisation in solution, of an iodide-bridged Ruâ Ir dimer is facilitating the development of a plausible, internally consistent model on which to base the catalysis.