A method which allows measurements of the photoemissive quantum yield of a Cs3Sb cathode of a photomultiplier is described. The method consists of an accurate control of the photocathode polarization to separate its eventual effects from the dependence of the quantum yield on the temperature. Measurements thus performed on four 56AVP phototubes have been interpreted as an up‐to‐now unknown variation of the quantum yield due to the temperature. The measurements suggest that the Cs3Sb could exist in two states, one of which, never noticed before, is stable only below (260±2)°K (the transition temperature), and which moreover is very sensitive to the absorbed energy. This low temperature state has a low quantum yield (for whitish‐blue light), which, depending on the absorbed energy, can be even five to seven times lower than the yield of the usual high temperature state of Cs3Sb.