We report the first direct, in situ measurements of the chemical composition of size-segregated atmospheric nanoparticles in the 6--15 nm diameter range. These measurements were made of ambient aerosol directly following nucleation events in Atlanta, Georgia, during the 2002 Aerosol Nucleation and Real-time Characterization Experiment (ANARChE). The recently developed Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) was used to make these measurements and featured a new inlet that delivers mass of charged and size-segregated nanoparticles at sufficiently high rates to enable analysis at a typical time resolution of 10 min. Measurements in both the positive and negative ion spectra revealed that particles formed recently from nucleation events have enhanced concentrations of ammonium and sulfate and that to within the uncertainty of our measurements, ammonium sulfate could account for all of the sampled nanoparticle mass. No other compounds were detected in the particles during these events. Concurrent measurements of particle hygroscopicity and volatility, made using a Nanometer Tandem Differential Mobility Analyzer, support the conclusion that ammonium and sulfate are primary components of these newly formed particles.