We discuss the reason of differences in results of measurements of the work function performed by the photoelectric and field emission method in heterogeneous carbonaceous materials. This effect is caused by the external electric fields that are applied in both methods as well as by the specific electronic structure of the investigated materials. Another effect of abnormal lowering (in comparison to work function of grphite, fullerenes and Ni) of the effective work function value (less than 3eV) is also discussed.