The increasing use of manufactured nanoparticles ensures these materials will make their way into the environment. Silver nanoparticles in particular, due to use in a wide range of applications, have the potential to get into water systems, e.g., drinking water systems, ground water systems, estuaries, and/or lakes. One important question is what is the chemical and physical state of these nanoparticles in water? Are they present as isolated particles, agglomerates or dissolved ions, as this will dictate their fate and transport. Furthermore, does the chemical and physical state of the nanoparticles change as a function of size or differ from micron-sized particles of similar composition? In this study, an electrospray atomizer coupled to a scanning mobility particle sizer (ES-SMPS) is used to investigate the state of silver nanoparticles in water and aqueous nitric acid environments. Over the range of pH values investigated, 0.5–6.5, silver nanoparticles with a bimodal primary particle size distribution with the most intense peak at 5.0 ± 7.4 nm, as determined from transmission electron microscopy (TEM), show distinct size distributions indicating agglomeration between pH 6.5 and 3 and isolated nanoparticles at pH values from 2.5 to 1. At the lowest pH investigated, pH 0.5, there are no peaks detected by the SMPS, indicating complete nanoparticle dissolution. Further analysis of the solution shows dissolved Ag ions at a pH of 0.5. Interestingly, silver nanoparticle dissolution shows size dependent behavior as larger, micron-sized silver particles show no dissolution at this pH. Environmental implications of these results are discussed.