This paper describes the colloidal interactions of preformed positively charged iron oxyhydroxide particles : zeta potential, coagulation, and flocculation in the presence of nonionic, cationic and anionic polyelectrolytes. In salt-coagulated systems, the zeta potential shown as a function of the pH and of the KCI salt concentration is the controlling factor in the formation of particle aggregates. In polymer-flocculated systems, a moderately charged anionic polyelectrolyte was used to adsorb onto particle surfaces and then induced the bridging flocculation of oppositely charged iron oxyhydroxide particles, in which maximum flocculation was observed near zero zeta potential. It is pointed out that the relative importance between polymer bridging and charge neutralization mechanism in the coagulation-flocculation processes of colloidal particles is pH dependent. Correlation of the magnitude of zeta potential with the interparticle interaction energy is simulated by using DLVO theory. Moreover, the present study extends the conventional analysis through considering the temporal variation of the fractional surface coverage of colloidal particles by polymers. The theoretical predictions on the stability coefficients are compared with the experimental results.