The electron affinity of oxide semiconductors (In2O3,SnO2, etc.) is a topic of current interest. These materials are capable of forming barrier devices on a number of semiconductors which exhibit good photovoltaic conversion efficiency. We will show that simple methods based on the electrical measurements of oxide‐semiconductor/base‐semiconductor systems often lead to incorrect results, due to the uncertainty in the Fermi‐level position in the oxide‐semiconductors and other interface parameters. To determine the true potential of a particular oxide‐semiconductor, other methods should be used to find the electron affinity.