A bulk membrane responsive to chloride has been developed containing a luminescent ruthenium complex placed in close proximity to the pH-sensitive absorber bromothymol blue (BTB), both contained in a plasticized poly(vinyl chloride) (PVC) membrane along with an anion-selective carrier. The response is based on the co-extraction of chloride along with protons from the aqueous sample into the PVC membrane and results in a large change in the absorbance of BTB. The absorption band of deprotonated BTB overlaps significantly with the emission band of the ruthenium complex so that radiationless energy transfer can take place and decay times are rather short (ca. 0.60 ms). If, however, chloride is co-extracted into the bulk along with a proton, the dye is converted into the yellow phenolic form. Hence, no energy transfer occurs and decay times are in the order of 1 ms. The signal changes can also be detected by measuring ¯uorescence intensity. The dynamic range can be adjusted via the sample pH and is from 30 to 180 mM chloride at pH 7.0 and thus covers the physiological range.