For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square-wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open-circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]-phenyl-C61-butyl acid methyl ester or [6,6]-phenyl-C71-butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.