Using THz time domain spectroscopy, we investigate ultrafast photoconductivity in two conjugated polymers, derivatives of poly(phenylene vinylene) (MEH-PPV) and polythiophene (RR-P3HT), to investigate free charge carrier dynamics. We find that the ultrafast photo-conductivity due to free charges is larger in the RR-P3HT than in the MEH-PPV sample. These charges are long-lived, remaining mobile for several nanoseconds, in contrast to the picosecond decay of free charges in MEH-PPV. By comparing our measured conductivities to charge mobilities inferred from microwave absorption measurements, we make estimates for the free carrier quantum yields. (c) 2006 Published by Elsevier B.V.