The accurate size determination of nanoparticles in solution is an important subject in nano/bio-technologies. However, interactions between particles induce a significant misinterpretation of the size determination by PFG-NMR method. In the sodium dodecyl sulfate (SDS) aqueous solution, we observed the significant change of the apparent size of PS-latex that depended on the concentration of SDS. In this Letter, accurate size determination of PS latex was carried out by extrapolation methods varying both the concentrations of the PS latex and the SDS in aqueous solution. The proposed method makes it possible to compare the determined sizes of nanoparticles in the liquid-phase by PFG-NMR to those by differential mobility analyzer (DMA) in the gas-phase. (c) 2008 Elsevier B. V. All rights reserved.