Absolute aggregation rate constants of anionic polystyrene sulfate latex particles were measured in the presence of cationic poly(vinylamine) by time-resolved simultaneous static and dynamic light scattering. The colloidal stability depends sensitively on the polymer dose, pH, and ionic strength. While the stability ratio is strongly dependent on the ionic strength, the electrophoretic mobility depends much more weakly on it, and therefore the latter measurements cannot be used as a reliable measure of the suspension stability. In the fast aggregation regime, the presence of the oppositely charged polyelectrolytes leads to an enhancement of the aggregation rate with respect to the one in the presence of an indifferent electrolyte. This acceleration probably originates from attractive electrostatic interactions between surface charge heterogeneities.