Mechanisms for the stabilization and flocculation of colloids have been indicated by microelectrophoresis measurements and electron-microscope observations with model colloids and polymeric flocculants. Zeta-potential (ζ) changes and details of floc structure were observed with silica and polystyrene latex colloids. Bridging fibers of polyamine-type flocculants appear to extend radially from the colloidal particles and vary in thickness from 20 to 300 Å. Charge neutralization and bridging may function simultaneously. Incremental additions of cationic flocculants produce gradual reduction in the negative ζ, and maximum flocculation is observed near zero ζ. Subsequent addition of flocculant reverses the potential and finally effects redispersion of the colloid. If incipient charge reversal is produced with a relatively low molecular weight cationic polymer, large flocs may then be formed on the addition of a high molecular weight anionic flocculant. Direct addition of an anionic polymeric flocculant to a negatively charged colloid may raise the negative ζ to a surprisingly high value and may thus effect stabilization instead of flocculation of the colloid.