Addition of chloride ions to boron bis(phenolate) 5 in dichloromethane solution produces a selective fluorescence decrease. The fluorescence change is believed to be caused by associative hydrogen bonding between the chloride ion and two boronic acid groups. While addition of fluoride ions to bis(phenolate) 5 generates a purple colorimetric response, the colorimetric response is caused by fluoride induced B-O bond cleavage and air oxidation of the phenolate anion formed by this dissociation.