In this article, we have investigated a temperature-rise fractionation procedure for poly(3-hexyl thophene) (P3HT) and poly(3-octyl thophene) (P3OT) that provides well-defined molecular weight (MW) fractions with improved molecular weight distributions (MWD) when compared with Soxhlet extraction. This process involves dispersing the material over C18-boned silica stationary phase in a jacketed column and using incremental rises in column temperature (T-col) to gradually improve solvent quality and selectively dissolve higher molecular weight samples with a narrow polydispersity (PDI). Fractionation of P3HT with Delta T-col = 5 degrees C in methylene chloride (MC) yielded 7 fractions ranging from M-p of 20 to 53 kg/mol with an average PDI of 1.80 compared with a mother sample of 3.10. Predominant recovery of P3HT was acquired for fractions with T-col > 20 degrees C (30 wt %). Subsequent separation of P3OT in methylene chloride, with a reduced Delta T-col of 3 degrees C per fraction, due to increased solubility from the longer alkyl chain, generated 8 fractions with a weight range of M-n = 22 to 57 kg/mol with an mean PDI of 1.23 with the mother sample having PDI = 2.34, demonstrating the tunability of this method (C) 2009 Wiley Periodicals, Inc J Polym Sci Part B Polym Phys 47, 2547-2555, 2009