The ability of meso-tetra(pentafluorophenyl)porpholactone (T(F)PL) and its Pt(II) complex [meso-tetra(pentafluorophenyl)porpholactonato]Pt(II) (T(F)PLPt) to function as optical high pH sensors is described. Under strongly alkaline or high methoxide conditions, their UV-vis spectra undergo dramatic and reversible red-shifts. The dynamic range for the sensor T(F)PLPt in solution is from pH 11.5 to 13.2. Using (1)H, (19)F, and (13)C NMR, UV-vis and IR spectroscopy, mass spectrometry, and the use of model compounds, the molecular origin of this optical shift is deduced to be a nucleophilic attack of OH(-)/MeO(-) on the lactone carbonyl of the chromophore, representing a novel mechanism for porphyrin-based sensors. The sensing compound was solubilized with Cremophor EL for use in aqueous solutions and embedded in polymer matrixes for testing as optical fiber-based optodes and planar sheet optode materials.