Acetone and methanol have been measured hourly at a rural mountain site in California for a full year, providing a unique data set for analyzing the factors controlling their seasonal cycles. Their mixing ratios showed clear maxima in summer and late spring respectively, and were mostly correlated with atmospheric temperature with highest values during heat waves. Lowest values occurred during winter coincident with the periods of lowest temperature (0.15 ppb for acetone and 0.4 ppb for methanol). The acetone to methanol ratio changed seasonally with a minimum in spring and maxima in late summer and fall, likely reflecting higher springtime methanol emissions. The ratio was highest during fall and winter, probably a result of secondary acetone production from diffuse anthropogenic sources. Large decreases of both gases were observed when frontal passages brought free tropospheric air of Pacific origin to the site as indicated by back trajectory analysis. Neither wet nor dry deposition could explain these periodic decreases. Our results confirm previous assessments showing large acetone and methanol abundances over rural areas indicative of large biogenic sources. Their high degree of correlation at all times suggests a strong link between their biogenic emission sources and only a small influence of anthropogenic sources, at least at this site. The free tropospheric background mixing ratios concur with published measurements from aircraft over the Pacific, and are consistent with a substantial, diffuse source from hydrocarbon oxidation.