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The sedimentation half-times t s of suspensions of polydisperse titania particles in aqueous Triton X-100 (TX100) solutions with concentrations c TX 100 ranging from 0.05 to 250 mM were measured. Three regions of TX100 concentrations with different settling behavior were observed. In region A, or for c TX 100 below its cmc (ca. 0.24 mM in water at 25 °C), the t s -values increase with increasing surfactant concentration. In region B, or for cmc ≤ c TX 100 < c * * , where c * * was determined to be 30 mM experimentally, the t s -values do not vary with the surfactant concentration. In region C, or for c TX 100 ≥ c * * , as c TX 100 increases, the t s -values increase. In regions A and B, little or no viscosity effect on the sedimentation was observed. By contrast, in region C, the viscosity of the micellar solution increases with increasing c TX 100 and accounts partially for the slower particle sedimentation rate. The surface densities of adsorbed TX100 molecules on the particle surfaces were measured to probe their effects on the steric interactions, which are responsible for the increase in the t s -values in region A. The maximum adsorbed densities on the particle surfaces, which are presumed to be hydrophilic, were found to be much smaller than those on other hydrophobic surfaces, as expected. Nonetheless, the adsorbed densities are high enough to induce strong steric interactions in region B, preventing the particles from agglomerating. In region C, the increase in viscosity slows down not only the particle sedimentation, but also the particle agglomeration, resulting in an apparently weaker micellar depletion effect. Our inference is that depletion-induced flocculation in region C is masked by the high solution viscosity. This inference is supported by several different lines of evidence. First, the once corrected half-times, t s I , which are the measured half-times divided by the relative viscosities, start to decrease at c TX 100 ≅ 160 mM, suggesting the formation of particle clusters, and reveals a micelle-induced depletion effect. The twice corrected half-times, t s II , which are t s I divided by the viscosity of the solution to account for the effect of viscosity on the agglomeration rate, are consistent with these inferences. In addition, the DLS data support our inference that the clusters in region C are flocs, which can be de-agglomerated with some mild agitation. A recently published model of agglomeration and sedimentation (Yang et al., 2016) is used to fit the data for estimating the effective Fuchs-Smoluchowski stability ratio W for flocculation. W -values are inferred to be over 10 6 at c TX 100 < 160 mM, but only ca. 500–1000 at higher concentrations, supporting the hypothesis of a depletion-induced flocculation that is masked by the solution viscosity. [ABSTRACT FROM AUTHOR]