The few-picosecond (ps) decay of terahertz (THz) photoconductivity typically observed for conjugated polymer:fullerene blends (at excitation fluencies ∼1015 photons/cm2 per pulse) is shown to be a result of charge pair annihilation for two polymer:PCBM blends. At a factor of 100 lower excitation density, the THz decay is in the hundreds of ps time scale, implying that very high carrier mobility (∼0.1 cm2 V–1 s–1) prevails for long time after charge formation, of importance for free charge formation in organic solar cells.