In conjugated polymers, the process of interconversion between singlet and triplet bound charge-pairs is important in determining the efficiency limits both for light-emitting diodes (LEDs) and for photovoltaics. We probe the kinetics of spin-mixing in charge-pairs by studying the formation of triplet excitons after photoexcitation of blends of poly( 9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with poly(9,9-dioctylfluorene-co-bis-N, N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) using excited-state transient absorption spectroscopy. We see clear evidence of triplet formation from long-lived charge-pairs during a period up to 100 ns after the excitation pulse, and we find that the intersystem crossing rate in these charge- pairs exceeds 2.0 x 10(-6) s(-1). (c) 2008 Elsevier B. V. All rights reserved.