A kind of fluorescent oligomeric pH indicator (fluorescein-formaldehyde product) was synthesized by the reaction of fluorescein with formaldehyde under alkaline conditions by a one-pot method. The synthesized product was fully characterized with Fourier transform infrared, H-1 NMR, ultraviolet-visible spectra, luminescence spectra, and matrix-assisted laser desorption/ionization time-of-flight mass spectra. The results indicated that the reaction was a typical phenol-formaldehyde reaction and that the fluorescein-formaldehyde product contained dimer, trimer, tetramer, and even a little octamer. Visible spectra and luminescence spectra showed that the fluorescein-formaldehyde product could still provide pH sensitivity similar to that of fluorescein. In addition, the oligomeric pH indicator immobilized poly(vinyl alcohol) membranes could be easily fabricated and achieved better long-term stability and fast equilibrium response. Thus, they are promising transparent membranes for optical pH sensors of a wide pH range (0.0-10.0) based on absorption and fluorescence. (c) 2005 Wiley Periodicals, Inc.