The hygroscopic growth factors of acidic sulfate nanoparticles relevant to atmospheric new-particle formation and growth were measured using a tandem nano-differential mobility analyzer (TnDMA). The dry diameters of the particles ranged from 5.5 to 53.2 nm. The growth factors progressively decreased for smaller dry particle diameters. For the largest particles, a model including the Kelvin effect and assuming fully acidic particles agreed well with the observed hygroscopic growth factors. For particles having dry diameters smaller than 36.1 nm, an expanded model based on a progressively increasing extent of neutralization for smaller particles and for higher relative humidity values explained the observations. Partial neutralization Occurred because of adventitious NH(3) in the experimental setup and was most significant for the lowest H(2)SO(4) mass loadings, corresponding to the smallest nanoparticles. The extent of neutralization reached as high as 0.50 for the smallest particles at 80% relative humidity. Alternative explanations such as inaccuracies or nanosize effects in the density or the surface tension of the particles could not explain the observations. These results show that the hygroscopic behavior of acidic sulfate nanoparticles is more sensitive to the extent of neutralization than to the other considered possible nanosize effects, at least for dry particle diameters as small as 5.5 nm. (C) 2009 Published by Elsevier Ltd.