The gel point of a colloidal unimolecular polymer (CUP) aqueous suspension was determined. The zero-shear viscosities of suspensions were measured by capillary viscometer and cone-and-plate rheometer. The relative viscosities were fit to the Krieger-Dougherty equation Mathematical expression to determine the maximum packing volume fraction φsymbolmax. The gel point was found to be much lower than the anticipated random close packing ( 0.63) or hexagonal packing ( 0.74). The gel point was attributed to the effect of surface water, and subsequently the thickness and density of water layer were calculated. The CUP results were compared with the rheological behavior of commercial waterborne suspension with particle size 25 and 77 nm. The packing volume fraction of colloids with different particle sizes was predicted considering the effect of surface water. [Figure not available: see fulltext.] © 2013 Springer Verlag Berlin Heidelberg.