Well-dispersed iridium(0) nanoparticles stabilized with the ionic liquid, trihexyltetradecylphosphonium methylsulfonate, [THTdP][MS], have been successfully prepared by reduction of the precursor hydridoiridium carborane, (Ph3P)(2)Ir(H)-(7,8-nido-C2B9H11). The iridium nanoparticles were found to be active catalysts for arylborylation, forming boric acids. The activity of the catalyst has been investigated as a function of the activating base, and reaction conditions. The highest yield of 91% was achieved in a microwave reactor using the base, tetra-2-pyridinylpyrazine, in the presence of [THTdP][MS]. The catalytic system could be recycled at least six times with less than a 0.5% loss of activity.