Abstract
From an experimental study of p‐type 
as a cathode for the photoelectrolysis of water, we have determined its flatband potential [+0.1V (SCE) at a pH of 9.2] and its bandgap (3.47 eV, indirect). The electron affinity of 
is only 1.4 eV, which is unusually small compared to most other oxides; this is due to the valence band being mainly nickel‐3d type instead of the usual oxygen‐2p type. The relevance of 3d valence bands to the performance of semiconducting photoelectrodes is discussed.