Ultrafiltration techniques (pore size of membrane below 100 nm) are widely used in chemical engineering, semiconductor, pharmaceutical, food and beverage industries. However, for small particles, which are more and more attracting interests, the pore size often does not correlate well with sieving characteristics of the ultra-membranes. This may cause serious issues during modeling and prediction of retention efficiencies. Herein, a series of liquid filtration experiments with unfavorable conditions were performed. PTFE membranes (50, 100 nm) and Nuclepore filters (50, 400 nm) were challenged with 1.7 nm manganese doped ZnS and 6.6 nm ZnO quantum dots (QDs), 12.4, 34.4 and 50 nm Au and 150 nm SiO2 nanoparticles. For larger and medium sized particles, sieving and eventually pore blockage phenomena were observed. In comparison, for small QDs, a high initial retention efficiency (> 0.4) in both filters was monitored, followed by a reduced efficiency with ongoing particle loading. This high initial retention of small nanoparticles was attributed to diffusion deposition rather than to sieving since the ratio of pore size to particle size was significantly high (up to 58). Our experimental results allow a basic understanding of the deposition mechanism of small nanoparticles (diffusion vs. sieving) in different filter structures. (C) 2015 Elsevier B.V. All rights reserved.