The critical flocculation concentration, CFC, of a polystyrene latex (PSL) dispersion was determined from the change in transmittance after adding an electrolyte solution. The apparent values of the CFC obtained from transmittance vs. electrolyte concentration curves decreased with increasing stand time. From the CFC value obtained at the shortest stand time and the experimentally determined outer Helmholtz plane potential, the Hamaker constant between PSL particles in water was calculated by considering particle aggregation in the secondary minimum of the potential energy curve. The Hamaker constant was verified by adhesion experiments: the adhesion of PSL particles to a quartz plate was explained in terms of the total potential energies of interaction calculated from the obtained Hamaker constant on the basis of the heterocoagulation theory.