The surface charge at the interface of a Au electrode with a KCIO4 solution was measured by in situ atomic force microscopy (AFM) using a modified cantilever with a charged SiO2 sphere. The effective charge determined by treating the AFM force curve was much smaller than that injected electrochemically, with the ratio of the effective/real surface charge to the electrochemical charge being below 10%. This large difference suggests that classical Gouy-Chapman-Stern (GCS) theory is inadequate to describe the diffuse electrical double layer. Ion correlation and ion condensation effects might account for the reduced surface charge. Additional experiments on the effect of electrolytes containing divalent species [Ca(NO3)(2) and Na2SO4] and the effect of adsorption of sodium dodecyl sulfate on the Au electrode provide additional evidence for such effects.