The interaction forces between silica surfaces with adsorbed layers of polyethyleneimine (PEI) were measured using colloidal probe microscopy as a function of solution conditions and PEI molecular weight. The surface coverage of polymer, as determined from optical reflectometry, was a significant factor influencing the extension of the chains away from the interface and also the development of adhesion between the surfaces. For the high molecular weight PEI (70 kDa), the adhesion passed through a maximum as a function of pH. The magnitude and position of this maximum was dependent on the surface excess of adsorbed PEI. The greatest adhesion was observed for the highest surface coverage of 0.88 mg m(-2) at pH 11 where the force-distance curves on approach indicated the presence of a significant steric layer. Furthermore, the forces on separation under these conditions indicated strong bridging of PEI chains across the interface contributing to the enhanced adhesion. However, at lower surface coverage, no bridging was observed but the adhesion was still significantly greater than in the absence of an adsorbed layer of PEI. The adhesion at lower surface coverage was indicative of a charge-patch mechanism. The measured values of adhesion correlated very well with the observed yield stress of concentrated dispersions of silica in the presence of adsorbed layers of PEI. Thus, the molecular mechanisms probed during surface forces measurements can be used to predict the ensemble behaviour of the many particles dispersed in an aqueous medium which is of particular importance in minerals processing.