Six room temperature ionic liquids (RTILs) comprised of a tetraalkylphosphonium cation (tridecylmethyphosphonium or trihexyltetradecylphosphonium) and a trihalide anion (Br(3)(-), BrCl(2)(-), or ClBr(2)(-)) have been prepared and characterized. Their ability to effect halogenation reactions with a variety of substrates has been explored. In general, halogenation reactions of alkenes proceed with high yields and stereo- and regioselectivities, whether performed in the absence or presence of a solvent (chloroform). Reactions of an alkyne and electrophilic substitution on an aromatic ring have been investigated as well. The facile preparation of the salts, their ease of handling, and the simplicity of product isolation should make these RTILs useful additions to the repertoire of halogenation reagents and the reagents of choice for specific transformations.