The cell-permeant pH indicator, 5-(and-6)-chloromethyl SNARF®-1, acetate has a pK_a of ~7.5, thus is useful for measuring pH changes between pH 7 and pH 8. Carboxy-SNARF®-1 exhibits a significant pH-dependent emission shift from yellow-orange to deep red fluorescence under acidic and basic conditions, respectively. This pH dependence allows the ratio of the fluorescence intensities from the dye at two emission wavelengths - typically 580 nm and 640 nm - to be used for quantitative determinations of pH. The mildly thiol-reactive chloromethyl moiety on this molecule should react with glutathione or other intracellular thiols to improve cellular retention of this substrate.