The surface forces apparatus was used to measure directly the interaction forces between mica-adsorbed quaternarized poly(2-vinylpyridine) (QP2VP) layers as a function of solution pH. The interaction is repulsive at large-surface separations and is dominated by a double layer interaction. At shorter range, electrosteric forces dominate until, at small-surface separations, attractive bridging forces lead to intersurface adhesion. The bridging attractive forces are attributed to both intersurface bridging and polyelectrolyte chain entanglement. These are extremely sensitive to the conformation of the adsorbed polyelectrolyte, which changes significantly with solution pH. Surface forces measured on compression of the adsorbed QP2VP layers do not clearly reflect changes in the adsorbed conformation and surface excess of this polyelectrolyte. Rather, the polyelectrolyte conformation is manifest more dramatically on measurement of adhesion, upon retraction/decompression of the surfaces from contact. There is a strong dependence of the adhesion between the polyelectrolyte layers on the compressive load, time in contact, and compression history since the molecular rearrangements required for chain entanglement and intersurface bridging occur on a long time scale, i.e., of the order of minutes. Under a small compressive load, the surfaces are closer together, facilitating segment-surface andsegment-segment interactions across the two interacting layers.