Most of the fluorescent pH probes work near neutral or acidic regions of the pH scale. In this work, two different fluorescent Schiff bases, chlorophenyl imino propenyl aniline (CPIPA) and nitro phenyl imino propen aniline (NPIPA), have been investigated for pH sensing in the alkaline region. Absorption and emission based spectral data, quantum yeild, fluorecence lifetime, photostability and acidity constant (pK(a)) of the Schiff bases were determined in conventional solvents and in PVC. The long wavelength excitable immobilized Schiff bases CPIPA (lambda(ex) = 556 nm) and NPIPA (lambda(ex) = 570 nm) exhibited absorption and emission based optical response to proton in the pH range of 8.0-12.0 and 7.0-12.0, respectively. Response of the CPIPA was fully reversible within the dynamic working range. the response times were between 3-13 min. A relative signal change of 95% and 96% have been achievedd for sensor dyes of CPIPA and NPIPA, respectively. The CPIPA displayed better fluorescence quantum yield (phi(F) = 3.7 x 10(-1)) and higher matrix compatibility compared to NPIPA (phi(F) = 1.6 x 10(-1)) in immobilized PVC. The CPIPA and NPIPA exhibited a slight cross sensitivity to the ions of Hg+ and Fe3+, respectively. (c) 2007 Elsevier B.V. All rights reserved.