Using transient absorption spectroscopy of 0.05 mM and 8 mM solutions of TIPS-Pn and TIPS-Tn in toluene, we have quantified an upper bound on the rate constant (lower bound on the time constant) of intersystem crossing (ISC) in these solutions as 3.08 × 10 6 s -1 (325 ns) and 8.50 × 10 6 s -1 (118 ns), respectively. Taking the ISC rate constants into account, we were also able to quantify the bimolecular singlet fission (SF) rate constants in these solutions as (2.6 ± 0.7) × 10 9 M -1 s -1 for TIPS-Pn, and (2.3 ± 0.9) × 10 9 M -1 s -1 for TIPS-Tn, where the range of values comes from the uncertainty in the ISC rate constant and in the T 1 basis spectra. We have pointed out some discrepancies in the literature, and shown that ISC can be overestimated by disregarding diffusion-limited SF in dilute solutions. Alternatively, SF can also be overestimated by not appropriately considering T 1 contributions from ISC. For systems in which SF is relatively slow, or is limited by diffusion through solution or exciton diffusion through a disordered material, the effect of ISC is particularly important. To make progress in the implementation of SF in photovoltaic devices (or in fact in other applications, such as quantum computing), it is important to accurately identify and quantify SF rate constants and yields, which would be impossible in many circumstances if ISC is not also considered.