Abstract The interaction and adhesion forces between illite particles and cleaved illite surface in aqueous solutions were measured using an atomic force microscope. Considering an extremely rough nature of the cleaved illite surfaces, a procedure was established to measure the forces between illite probes and raised spots on the cleaved surface. The measured long-range interaction forces were well-described by the classic DLVO theory. The effect of solution pH, divalent cation and flocculant addition on the forces and stability of illite suspensions was studied. The results showed a strong adhesion force and a weak repulsive (or even attractive) long-range force with calcium and magnesium addition as well as lower solution pH. These conditions are favourable for the coagulation of illite particles. With the addition of HPAM, a partially hydrolyzed polyacrylamide flocculant, a strong bridging adhesion force was measured. The measured forces are consistent with the observed stability of illite suspensions. The quantitative description of illite–illite interactions provided fundamental insights into the mechanism of bitumen extraction and tailings treatment and justified the industrial use of gypsum and polymer flocculants in oil sands processing. As illite is extensively distributed in the Earth's crust, fundamental understanding of illite–illite interactions would also be beneficial to many other industrial and natural processes.