Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N-2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets r(g) was r(g)=13 (+/-1) nm, the polydispersity ln sigma(r) was ln sigma(r)=0.19 (+/-0.07), and the number density N was N=(2+/-0.2).10(11) cm(-3). The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering spectra as a function of the isotopic composition gave further evidence that the binary droplets exhibit ideal mixing behavior. Because both the stagnation temperature T-0 and the location of onset were fixed, the temperature corresponding to the maximum nucleation rate was constant at T-J max=229 (+/-1) K. Thus, the experiments let us estimate the isothermal peak nucleation rates as a function of the isotopic composition. The nucleation rates were found to be essentially constant with J(max) equal to (3.6+/-0.5).10(16) cm(-3) s(-1) at a mean supersaturation of 44 (+/-3). (C) 2003 American Institute of Physics.