Aggregative stability of a polydisperse zirconium oxide sol with average sizes of primary particles and aggregates equal to 24 and 95 nm, respectively, has been studied within a wide range of sodium chloride concentrations (3 × 10−3–1 M) by turbidimetry and dynamic light scattering. The experiments have been performed at pH 4.2 and 5.6, when sol particles are positively charged, and at pH 11, when they are negatively charged. The regions of weak and intense slow coagulation, as well as the region of fast coagulation, have been
determined and characterized. It has been shown that, in most cases, the thresholds of the intense slow coagulation determined from the concentration dependences of the optical density and the average particle size
coincide with each other within the experiment error. Assumptions have been brought out on the coagulation mechanism and its changes upon the passages between the coagulation regions, as well as on the role of the ratios between the particle sizes of separate fractions and their amounts in the sol.