Triplet-exciton formation through intersystem crossing of photogenerated singlet excitons in fullerene acceptors can compete with charge generation in organic photovoltaic diodes. This article reports the intersystem crossing timescale (tau(ISC)) of the most commonly used fullerene acceptors, PC60BM and PC70BM, in solutions and in spin-coated films. These times are on the nanosecond timescale, and are longer than the characteristic times for charge generation (tau(d)).