Rate constants for important elementary reactions of 2,4-dimethyl-1,2,4-triazol-3-ylidene borane (1) and its derived boryl radical have been measured in the presence and absence of water. Reactions including H-abstraction by tBuO•, and onward reactions of the derived radical with methyl acrylate (addition), iodopropane (halogen abstraction), and an iodonium salt (oxidation) are unaffected by the presence of a high concentration (1.5 M) of water. Extending these results, borane (1) has been found to be an efficient new co-initiator for the visible light photopolymerization of monomers in the presence of both air and water.