We report Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of the composition of ambient 10-33 nm diameter particles formed from nucleation on 16 March 2006 in Tecamac, Mexico during the Megacity Initiative: Local and Global Research Observations (MILAGRO) field study. On this day, nucleated particles contained far more organics than sulfates: average ion molar ratios with measurement uncertainties for nitrate, organics and sulfur species were 6±2%, 84±5%, and 10±1%, respectively. The measured organic species include nitrogen-containing organic compounds, organic acids, and hydroxy organic acids. Independent calculations show that sulfuric acid condensation could have accounted for only 10±2% of the growth that was observed on this day, which is consistent with the TDCIMS measurements of composition. It follows that organic compounds play a dominant role the high growth rates that were observed.