When a nitrobenzene (NB) phase came in quiescent contact with a water phase, water droplets were formed spontaneously near the oil/water interfaces (K. Aoki, M. Li, J. Chen, T. Nishiumi, Electrochem. Commn. 11 (2009) 239). We reported here quantitative data of the NB-droplets by use of UV-absorbance, dynamic light scattering (DLS) and voltammetry with a help of ferrocene. The supernatant separated spontaneously from the oil-water mixture contained 15.5 mmol dm(-3) evaluated by UV spectra, whereas the centrifuged supernatant did 9.3 mM. The difference suggested the presence of NB-droplets, the diameter of which ranged 0.15-0.5 mu m obtained by DLS. Ferrocene was dissolved deliberately in the aqueous solution and the NB solution up to saturation. The voltammograms in the ultrasonicated supernatant exhibited diffusion-controlled redox peaks of ferrocene, which should be supplied from both dissolved ferrocene and ferrocene-dissolved NB droplets. The former was 1/3.6 times of the latter by the comparison with the current of ferrocene-saturated aqueous solution. Applying the expression for the diffusion-controlled peak current of a big particle, we estimated the number concentration of NB droplet to be 1.1 x 10(14) dm(-3). This value is equivalent to the average distance, L = 2.1 mu m, between neighboring two droplets, corresponding to the diffusional traveling time, L-2/D = 1.3 s, which would be long enough for collision of droplets to stabilize the emulsions. (C) 2011 Elsevier B.V. All rights reserved.