X-ray photoelectron spectroscopy (XPS), an essentially macroscopic probe, is used to analyze mesoscopic systems at a lateral resolution given by the substrate structure. The method is based on controlled differential charging of multi-component surfaces, using a simple, commonly available XPS function, the electron flood gun. This new approach is applied here to a novel composite surface comprising SiO2 clusters on a {111} gold substrate, onto which different molecules are self-assembled to form a mixed organic monolayer. The method allows direct correlation of adsorbed molecules with surface sites, by analyzing XPS line shifts, which reflect local potential variations resulting from differential surface conductivity. This provides a powerful tool for resolving complex ultrathin films on heterogeneous substrates, on a length scale much smaller than the probe size.