Hypothesis: The debate as to whether nanoparticles that are formed upon mixing ethanol and water are nanobubbles or other nanoparticles has continued over the past decade. In this work, we test the hypothesis that long lived bulk nanobubbles are produced upon mixing ethanol and water, using techniques that probe the density and the pressure response of the nanoparticles. Experiments: Nanoparticles were generated spontaneously upon mixing high-purity ethanol and highpurity water. The size distribution of these nanoparticles was obtained using nanoparticle tracking analysis. The mean density of the nanoparticles was determined using resonant mass measurement, and the response of the nanoparticles to the application of external pressure was measured using dynamic light scattering. Findings: The ethanol-water mixture was found to produce only positively buoyant particles, with a mean density of 0.91 +/- 0.01 g/cm(3), and the external pressure had only a minimal effect on the size of these nanoparticles. Degassing the solvents before mixing led to a significant reduction in the number of nanoparticles produced. Allowing the solutions to re-gas restored their ability to produce nanoparticles. These experiments reveal that ethanol-water mixing produces nanoparticles that result from the accumulation of material at the interface of dissolving bubbles. (C) 2019 Elsevier Inc. All rights reserved.