Biogenic volatile organic compounds (BVOC) were measured at Azusa and at either Pine Mountain or Mount Baldy, elevated sites 11 km north and 25 km northeast of Azusa, respectively, during four intensive sampling periods of the 1997 Southern California Ozone Study. During the sampling periods there was a consistent pattern of isolation of the mountain sites from valley air masses at night, followed by transport of valley air with elevated levels of O3 and NOx to the mountain sites as the mixing height increased throughout the day. Isoprene was the dominant BVOC at the mountain sites with afternoon concentrations reaching 2 ppbv, and its decrease to a low mixing ratio after sunset was attributed to reaction with NO3 radicals. At Azusa the BVOC mixing ratios were highest in the morning with the concentrations of monoterpenes and of methacrolein (MACR) and methyl vinyl ketone (MVK), isoprene photooxidation products, generally exceeding the maximum isoprene measured at Azusa. The high daytime ratio of (MVK+MACR)/isoprene suggested that nighttime drainage flows into Azusa, from elevated sites where the isoprene was depleted by chemical reaction, may have been responsible for much of the isoprene and its photoxidation products. The data also indicated local isoprene sources at Azusa and a possible contribution of MVK and MACR from vehicle emissions. Instances of high mixing ratios of limonene at Azusa suggested an intermittent anthropogenic source. During this study, particularly in early morning, BVOC are calculated to make a significant contribution to peroxy radical formation at Azusa. ¿ 2001 American Geophysical Union