The open-circuit voltage (V-OC) of an organic solar cell is limited by the donor-acceptor material system. The effective gap E-g(eff) between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V-OC, which is only reached for T 0 K. This relation is confirmed for a number of small-molecule bulk heterojunction p-i-n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V-OC is identical to E-g(eff). Furthermore, the influence of the hole transport material in a p-doped hole transport layer and the donor-acceptor mixing ratio on this limit V-0 is found to be negligible. Varying the active material system, the quantitative relation between V-0 and E-g(eff) is found to be identity. A comparison of V-0 in a series of nine different donor-acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material.