Pendant carboxyl groups were introduced into donor-acceptor polyfluorene carrying benzothiadiazole as an acceptor moiety. The emission color from its solution was different depending on the solvent. While the methanol solution of the polymer exhibited blue emission, yellow emission was observed from its tetrahydrofuran solution. Influence of solvent polarity, solution concentration, temperature, and addition of acid on photoluminescence properties indicated that the observed change in emission color was due to polymer aggregation coming from inter and/or intramolecular interaction between pendant carboxyl groups. Polyfluorene carrying benzothiadiazole moiety with pendant amino groups also exhibited emission color change and was useful as a fluorimetric sensor for polyacid.