A proof of principle experiment has been performed for the in situ measurement of the work function in vacuum conditions (~10−7 torr) relevant to cesium-seeded negative ion sources for fusion applications. The work function is measured using the principle of the photoelectric effect. The performance of the work function measurement setup and temperature-controlled cesium (Cs) oven has been characterized using various diagnostics such as a quartz crystal microbalance, a surface ionization detector and pressure measurements. The background impurities responsible for work function degradation are detected using the residual gas analyzer. The work function (⩽3.06 eV) and Cs flux (~1012 atoms s−1) have been measured directly at the sample surface during controlled Cs evaporation using a novel, inexpensive and portable design of the cathode–anode assembly.