The chemical compositions of single aerosol particles larger than 0.3 μm optical diameter were measured at Idaho Hill using a laser ionization mass spectrometer. The mass spectrometer can analyze either positive or negative ions; this paper covers the positive ion results. Potassium, iron, and organics were among the most common positive ions in the spectra of particles at Idaho Hill. Organic ions were the largest contributor to the ion current from accumulation mode particles and were also significant in larger particles. Iron was present mostly in coarse mode particles. Potassium was bimodal, with large amounts of K in coarse mode particles and large amounts in particles near 0.3 μm in west winds but not in east winds. Boron was observed in a number of particles. A wide variety of metals were also observed including lead, tin, and vanadium. Lead was less common in polluted air. Silver, copper, zinc, nickel, molybdenum, rhenium, and thallium were also observed in a few particles. Ammonium ions and cluster ions were more abundant in upslope conditions. Amines seem to be evident in many of the spectra, especially in east winds. Elemental carbon was observed in 2 to 3% of the particles. Almost all of the particles contained some nonvolatile material. ¿ 1997 American Geophysical Union