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We report on the use of Kelvin probe force microscopy in measuring the shift of the contact potential difference of micron-scale areas. The experimental results provide important information required for understanding and modeling the electrical characteristics of chemically sensitive field-effect transistors (ChemFETs). The temporal evolution in the shift of the contact potential difference of chemically sensitive monolayers of free-base porphyrin and zinc-porphyrin on exposure to pyridine gas was studied and their different behavior observed. The Kelvin probe force microscopy data on nanometer-scale areas were in agreement with those obtained with a conventional Kelvin probe on centimeter-scale areas. The accuracy of the measured shift in contact potential difference upon exposure to trace amounts of gas indicates the utility of Kelvin probe force microscopy as a means to characterize the operation of exposed-gate ChemFETs.