Precipitation chemistry measurements have been made at many California locations, with recorded extreme pH values as low as 2.89 from Pasadena and 3.5 from geologically sensitive Sequoia National Park. However, in northern areas of the state experiencing good air quality, very little acid deposition occurs. California's acid precipitation is apparently locally produced, since there are no major pollution source regions upwind of the state. This is attributable to the extremely rapid rates observed for conversion of NOx to nitric acid and SO2 to sulfuric acid in the atmosphere. Recent monitoring data illustrate extreme variations in precipitation chemistry from adjacent sites and even at the same location from one year to the next. Enrichment factor calculations show that in contrast with data from other locations in the world, nitric acid dominates over sulfuric acid in California's rainfall in many locations. Measurements of total atmospheric deposition from Orange County in 1967–1968 show values of highest acidity near stationary point sources and during the winter months, when wind-blown dust deposition was minimal. It is estimated that during this period, only 18 percent of the total sulfate deposition in Orange County was in the form of wet deposition, emphasizing the importance of dry deposition in the Los Angeles area. The significance of atmospheric acidity in fogs and dews in California is discussed, as well as current research efforts sponsored by the California Air Resources Board.