Our in situ small angle X-ray scattering (SAXS) measurements yield an unprecedented and detailed view of rapidly evolving H(2)O nanodroplets formed in supersonic nozzles. The SAXS experiments produce spectra in a few seconds that are comparable to small angle neutron scattering (SANS) spectra requiring several hours of integration time and the use of deuterated compounds. These measurements now make it possible to quantitatively determine the maximum nucleation and growth rates of small droplets formed under conditions that are far from equilibrium. Particle growth is directly followed from about 10 micros to 100 micros after particle formation with growth rates of approximately 0.2 to 0.02 nm micros(-1). The peak H(2)O nucleation rates lie between 10(17) and 10(18) cm(-3) s(-1).