New organo-soluble and blue-emissive fluorene-based polyoxadiazoles, PFOx and PFOxEH, exhibit colorimetric and fluorescent acid-sensory properties to a strong acid due to the weak basicity of imine-type nitrogen atom(s) in an 1,3,4-oxadiazole ring. To further investigate their protonation with an acid, H-1 NMR spectroscopy and AMI semiempirical quantum-mechanics are utilized as well as UV-visible absorption and fluorescence spectroscopies. These investigations indicate the protonation does not proceed thermodynamically but kinetically at the initial stage of the complex formation of an acid and an oxadiazole ring in the polymer backbone. (C) 2003 Elsevier Science Ltd. All rights reserved.