A whispering-gallery mode (WGM) optical resonance sensor for volatile organic compounds (VOCs) is developed from polystyrene (PS) microspheres doped with fluorescent beta-cyano-appended oligo(p-phenylenevinylene) (beta-COPV). The beta-COPV-doped PS microspheres (MSCOPV) are formed by the miniemulsion method in a binary solvent. MSCOPV expand upon permeation of VOCs into the PS matrix and exhibit a spectral shift of the WGM resonance peak. The permeation efficiency is highly dependent on the chemical affinity between the analyte and the polymer matrix, with exceptionally high selectivity toward aromatic hydrocarbons such as benzene, toluene, and xylenes (BTXs). The high selectivity and sensitivity of MSCOPV are in clear contrast to those of conventional WGM sensors that just detect VOCs nonpreferentially through adsorption onto the surface.