Long‐wavelength photoemission current from Cs and O2 coadsorbed W (100) surfaces has been measured as a function of the amount of Cs and O2 deposition. Auger, work‐function, and LEED measurements were used to monitor the exact amount of Cs and O2. We found that for different initial oxygen exposure a distinct maximum of photoemission efficiency exists at Cs coverage of ϕCs=0.229±0.3 (2.9×1014 Cs atoms/cm2), which is near but not at the well‐known work‐function minimum (ϕCs=0.25). The maximum photocurrent increases as the initial oxygen exposure increases. At optimum peak, the photoemission efficiencies are typically three to five times that of other Cs and O2 coverages.