Fluorescence quenching dynamics were studied in poly(9,9-dioctylfluorene) films with a wide range of beta-phase concentrations. Measurements of time-resolved fluorescence and fluorescence quantum yield showed that the radiative decay rate was independent of the beta phase content, whereas the nonrachative decay rate was found to be higher in beta-phase rich films. The trend was analyzed in terms of diffusion-mediated excitation energy transfer to quenchers. The concentration of fluorescence quenchers is found to be less than 0.01 % of the number of numeric: repeat units and independent of the fraction of the film in the beta phase. The results suggest that fluorescence quenching is predominantly by chemical defects rather than by excimer formation.