The absolute electrode potential in solid state electrochemistry is discussed, defined and measured. The term "absolute" potential denotes an electrode potential not based on another reference electrode system but to a given reference electronic energy taken as zero. This is important because then the energy scale of electrochemical systems can be directly compared with that of solid/gas or solid/vacuum interfaces. A two-Kelvin probe arrangement was used to measure for the first time in situ the work functions, phi, of the gas-exposed surfaces of porous Pt, Au and Ag working and reference electrodes, exposed to O-2-He, H-2-He and O-2-H-2 mixtures, and deposited on 8% Y2O3-stabilized-ZrO2 (YSZ) in a three-electrode solid electrolyte cell. It was found that at temperatures above 600 K, the potential difference, U-WR, between the working (W) and reference (R) electrode reflects the difference in the actual, spillover and adsorption-modi fled, work functions, phi(W) and phi(R) of the two electrodes: eU(WR) = phi(W) -phi(R) (1) This equation, typically valid over 0.8-1 V wide U-WR ranges, was found to hold for any combination of the Pt, Au and Ag electrodes. It is consistent with the previously reported equation: eDeltaU(WR) = Deltaphi(W) (2) also confirmed here. This is due to the creation via ion spillover of an effective electrochemical double layer on the gas-exposed electrode surfaces in solid electrolyte cells, which is similar to the double layer of emersed electrodes in aqueous electrochemistry, Eq. (1) allows the definition of a natural absolute electrode potential U-O2(abs) in solid state electrochemistry from: U-O2(abs) = phi/e (3) where phi is the work function of the gas-exposed electrode surface of the metal (any metal) electrode in contact with the YSZ solid electrolyte. It expresses the energy of "solvation" of an electron from vacuum to the Fermi level of the solid electrolyte, The value U-O2(0)(abs) = 5.14(+/- 0.05) V was determined as the standard U-O2(abs) value for YSZ at p(O2) = I bar and T = 673 K.