We use an improved setup for deducing quantitative surface potential values by means of frequency modulated Kelvin-probe force microscopy (FM-KPFM). This method is sensitive to the electrostatic force gradient rather than the absolute force probed in KPFM so far, and therefore provides both a higher lateral resolution and quantitative values. Furthermore, FM-KPFM allows using cantilevers with high spring constants which even favors both the stability and increased topographic resolution. Here, we apply FM-KPFM to deduce interfacial electrical properties of the sub-monolayer coverage of three adsorbates on metal substrates: lithium chloride films, Copper-porphyrines, and C₆₀ molecules.