An improved FEM for probe hole measurements and a convenient method for the evaluation of probe hole data have been developed during studies of coadsorption of oxygen on tungsten and oxygen and/or cesium on molybdenum. The entire emission pattern is accessible thereby for the investigation of adsorption parameters. An arbitrary plane can be chosen for examination by means of a manipulator. The changes in the field emission current during adsorption processes are recorded as a function of time. From these plots the corresponding changes in work function are derived by a special evaluation method. Time required for measuring and evaluating FEM data is considerably reduced and an improvement in accuracy is achieved as compared to the conventional method of drawing FN plots. The FEM, the method of evaluation, and some characteristic data obtained by this technique are presented.