A single-ultrafine-particle mass spectrometer was deployed for size-resolved ultrafine aerosol composition measurements during the winter of 2002--2003 in College Station, Texas. Three separate experiments were held between December and March with 6 week intervals. Almost 128,000 mass spectra, corresponding to particles with aerodynamic diameters between 30 and 300 nm, were collected and classified. Fifteen statistically significant classes were identified and are discussed in this paper. Nitrate, potassium, carbon, and silicon/silicon oxide were the most frequently observed ions. Nitrate was present in most of the particles, probably because of the agricultural activity in the vicinity of the sampling site. The nitrate detection frequency was found to be sensitive to the ambient temperature and relative humidity. Another particle class, identified as an amine, exhibited a strong relative humidity dependence, appearing only during periods of low relative humidity. There is evidence that some of the detected particles originated from the large urban centers and were coated with nitrate, sulfate, and organics during transport.