The study of the interaction between electrolytes and nonionic surfactants was extended to the micellar properties of octoxynol 9 at 25 degrees C. Density, surface tension, viscosity, and static and dynamic light scattering were measured in water (where dilute octoxynol solutions had a cloud point of 64 degrees C), in a salting-out electrolyte, 0.40 m Na2SO4 (cloud point lowering = 28 degrees C), and in the salting-in electrolytes 1.80 m NaSCN (cloud point increase = 27 degrees C) and 2.00 m Mg(NO3)(2) (cloud point increase = 14 degrees C), The intrinsic viscosity of octoxynol in Na2SO4 was twice as large as in water and NaSCN, the diffusion coefficient was 2.5 times smaller, the micellar molecular weight was 5 and 14 times larger than in water and NaSCN, respectively, and the micellar hydration was at least twice as large. The micellar properties in Mg(NO3)(2) differed from those in the other three media : The partial specific volumes in the latter were independent of surfactant concentration and had comparable values, while decreasing significantly with increasing surfactant concentration in the former. The micellar molecular weights and aggregation numbers were nearly the same in Mg(NO3)(2) and NaSCN and amounted to approximate to 1/3 of the corresponding values in water; the intrinsic viscosities were 98 and 83% of that in water, respectively, while the micellar diffusion coefficients at infinite dilution were 59 and 120%. The apparent diffusion coefficients in Mg(NO3)(2) increased slightly with increasing surfactant concentration while decreasing in the other three media. Only in Mg(NO3)(2) did the micellar hydration increase with increasing surfactant concentration, These observations point to micellar binding of Mg(NO3)(2), presumably via complexation of Mg2+ ions by the ether oxygens of the micellar polyoxyethylene moieties, which rendered the surfactant somewhat cationic.