The aggregation stability of tungsten oxide hydrosols is studied by the photometric and electrooptical methods. Coagulation thresholds determined in solutions of 1 : 1, 2 : 1, and 3 : 1 electrolytes obey the Schulze-Hardy rule. The coagulation is shown to occur by the barrier mechanism predominantly through adhering of small particles to large ones without considerable changes in the sizes of the latter.