A report is given of a series of surface force measurements between molecularly smooth mica surfaces in solutions of sodium dodecyl sulfate (SDS) and sodium polystyrene sulfonate (NaPSS). The micelles and polyions act as coions to the mica surface. This allows an investigation of the effect of large multivalent coions on the range and nature of the double-layer forces. Below the critical micelle concentration (CMC) of SDS, it is found that the forces are well described by assuming the SDS to be completely dissociated. Above the CMC, the micelles together with their effectively “bound” counterions do not contribute to the electrostatic screening. In NaPSS solutions, at constant monomolar concentration (i.e., concentration of polymer segments), the distance range of the double-layer forces increases with increasing chain length. Given that the bulk counterion activity at constant monomolar segment concentration is independent of the chain length, this observation strongly suggests that a polyelectrolyte depletion (together with a concurrent counterion depletion) occurs between the surfaces, which becomes enhanced with increasing molecular weight. Double-layer forces in the presence of divalent counterions increase in magnitude on addition of NaPSS. This is attributed to preferential counterion condensation on the polyelectrolyte backbone: depleted NaPSS effectively scavenges divalent ions from the ionic double-layers between the surfaces and releases a corresponding concentration of previously depleted monovalent counterions.