Time-resolved dynamic light scattering was used to investigate the influence of pH and adsorbed humic acid on the coagulation kinetics of kaolinite clay colloids. Stability ratios, defined as the experimental fast coagulation rate divided by the coagulation rate of the sample under consideration, were determined from the initial increases in average hydrodynamic radius with time. Pure kaolinite exhibited fast coagulation (stability ratio W = 1) at all ionic strengths (0.001 to 1 M NaClO4) when suspension pH was below 5.8. Above pH 5.8, coagulation rates were significantly reduced (W > 1) and W exhibited pronounced ionic strength dependence. The observed behavior is explained by edge-to-face heterocoagulation by electrostatic attractive forces at low pH and electrostatic stabilization at high pH. The transition in coagulation behavior occurs at the point of zero charge (PZC) of the kaolinite edge surfaces near pH 5.8. Small additions of humic acid at pH 4 resulted in edge charge reversal from positive to negative and substantially reduced coagulation rates (W > 1); similar behavior was observed at pH 6. Relating the observed stability ratios W to zeta potentials suggests that at low to medium ionic strength (less than or equal to 0.01 M NaClO4) the stabilizing effect of adsorbed humic acid is purely electrostatic. However, at high ionic strength (greater than or equal to 0.1 M) steric stabilization may also play a role.