Chemical analyses and qualitative observations during the processing of S‐20 photocathodes indicate that the cesium produces a bulk as well as a surface effect. Quantitative confirmation for a bulk effect is provided by x‐ray studies of Na2KSb and (Cs)Na2KSb. It was found that addition of Cs increases the lattice constant from 7.727±0.003 to 7.745±0.004 Å. It is suggested that the larger lattice constant leads to a greater excess of Sb in the lattice and thereby to enhancedp‐type doping and band bending.