Carbon films obtained using the spray pyrolysis technique are used in the construction of humidity sensitive resistors and capacitors. Both the electronic and surface properties of these films are dependent on several factors such as the carbonization temperature and duration. An addition of sodium containing salts (sodium acetate) to the basic precursor i.e., furfuryl alcohol, produces carbon films that have very specific surface properties. X-ray photoelectron spectroscopy studies revealed that the sodium atoms are located in the mother carbon network. These sodium atoms are associated with oxygen forming structures corresponding to Na2O. These spots act as centres of durable CO2 adsorption. This results in an intensive oxidation of the surface leading to a state where the surface layer contains more than 45% oxygen. Such an intensively oxidized surface is highly hydrophilic which results in an intensive water vapour adsorption that follows an I-type adsorption isotherm. The electronic conduction in the sodium modified carbon films is extremely sensitive to water adsorption and it increases by thousands of times after an exposure to humid air.