Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C9H19C(CH3)(2)SH]. Rate constants for reaction of the phenylthiyl (PhS center dot) radical with two NHC-boranes have been measured to be similar to 10(8) M-1 s(-1) by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.