We show that the Kelvin probe method (KPM) is well suited for in situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive, and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc-F16) are presented. The KPM results show that a 2.5% admixture of ZnPc-F16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc-F16 has acceptorlike properties relative to ZnPc. © 1996 American Institute of Physics.