Spin-cast films of a pentiptycene-derived phenyleneethynylene polymer, 2, display a fast fluorescence response (seconds) to vapors of 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 1,4-benzoquinone (BQ). The fluorescence attenuation of 2 is dependent on the time of exposure to these quenchers and on the thickness of thin films. Thinner films show a better response to TNT and DNT but the opposite is true for BQ. Such differences are attributed to different polymer-analyte interactions. For comparison, corresponding studies on an alkoxy-substituted phenylenethynylene polymer, 3, were also carried out. The results indicate that 2 is superior to 3 as a fluorescent chemosensor in terms of sensitivity, selectivity, solvent soly. and solvent stability. [on SciFinder(R)]