Using subpicosecond photoinduced absorption, we monitor the evolution of singlet excitons in solutions of regiorandom poly(3-octylthiophene) (P3OT), regioregular poly(3-hexylthiophene) (P3HT), and a well-defined alpha-oligothiophene with 12 repeat units (T-12) We find a luminescence lifetime of 500 ps, and we observe the intersystem crossing from the singlet manifold to the triplet manifold, with a rate constant of k(ISC)(-1)=1.2 ns. By measuring the quantum efficiency of luminescence, we estimate an intrinsic lifetime of 2 ns for the singlet excitons, and a nonradiative decay rate of k(nr)(-1)=1.5 ns. We find no difference in the intersystem crossing time for the different thiophene derivatives, implying the intersystem crossing rate is determined primarily by the relatively large spin-orbit interaction due to the sulfur heteroatom and not by chain defects, chain ends, or effects due to the side groups. In addition, we find that for t<50 ps a fraction of the photoexcitations undergo one-dimensional diffusion limited recombination.