The phase diagram of the system composed of H2O and tributylhexadecylphosphonium bromide (CTBPBr) has been investigated and the phase boundaries determined. CTBPBr solutions undergo critical demixing phenomena and separate into two fluid phases at temperatures close to 30 degrees C. Added alkali bromides have some effect on the phase boundaries, which are shifted to lower temperatures, in proportion to the amount of added salt. Additional studies were performed on tributylhexadecylphosphonium sulfate ((CTBP)(2)SO4) and nitrate (CTBPNO3). In agreement with the behavior predicted by the empirical Hofmeister series, the former does not show the occurrence of consolute boundaries but forms a liquid crystalline phase. The latter is nearly insoluble and transforms from a waxy solid in H2O dispersion to a biphasic fluid system, on increasing temperature. To analyze the role of the counterion, we performed a kinetic investigation of the cyclization reaction of 2-(3-bromopropyloxy)phenol (PhBr7) in dilute aqueous solutions of (CTBP)(2)SO4 and CTBPBr. Evidence of counterion specificity was observed from kinetics.