The gas-phase electron affinity (EA) of phenyl–C61–butyric acid methyl ester (PCBM), one of the best-performing electron acceptors in organic photovoltaic devices, was measured by low-temperature photoelectron spectroscopy for the first time. The obtained value of 2.63(1) eV is only ca. 0.05 eV lower than that of C60 (2.683(8) eV), compared to a 0.09 V difference in their E1/2 values measured in this work by cyclic voltammetry. Literature E(LUMO) values for PCBM that are typically estimated from cyclic voltammetry and commonly used as a quantitative measure of acceptor properties are dispersed over a wide range between −4.38 and −3.62 eV; the reasons for such a huge discrepancy are analyzed here, and a protocol for reliable and consistent estimations of relative fullerene-based acceptor strength in solution is proposed.