Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β-phase chains using time-resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time-independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time-dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β-phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low-energy sites. The annihilation rate in the mixed-phase films increases with increasing fraction of β-phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersedβ-phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line-dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy andβ-phase chromophores are intimately mixed rather than clustered or phase-separated.