A novel optical sensor membrane for the detection of mercury(II) (Hg(II)) is presented. The sensing layer is composed of plasticized poly (vinyl chloride) (PVC) containing a lipophilic boratesalt as areagent for Hg(II), and theamphiphilic oxacarbocyanine dye (DiOC₁₆(3)) as the optical transducer. The sensing scheme is based on the decomposition of the borate anion in the presence of Hg(II). Since this decomposition cannot be monitored optically, the borate anion is coupled to a fluorescent oxacarbocyanine cation to form a lipophilic ion pair. The decomposition of the borate anion leads to the formation of non-fluorescent dye aggregates which can be followed fluorometrically. The extraction of Hg(II) from the water phase into the membrane leads to a continuous and irreversible decrease in fluorescence intensity, so that a kinetic approach is used for calibration. An exposure time of 30 min allows for the determination of Hg(II) at 100 nM concentration levels. The membranes respond to Hg(II) with very high selectivity over other metal ions, and the pH effects are found to be negligible. The membrane can be regenerated by exposing it to a solution containing the same borate. Effects of substituted tetraphenylborates on the response of the membranes are reported.