Isoprene and its oxidation products, methyl vinyl ketone (MVK) and methacrolein (MACR), were measured in a semirural environment that was occasionally heavily impacted by urban emissions. At this site, isoprene was the most important hydrocarbon in terms of kOH⋅, but the aldehydes HCHO and CH3CHO also appear to be very important. The local isoprene photochemistry appears to be occasionally enhanced in NOx-rich urban plumes that are advected to the site over intermediate forested land. When O3 was being rapidly produced in urban plumes advected to this forested site, isoprene was found to contribute ≈28% of the total ozone production. We observe that many of the peaks in isoprene oxidation products at this surface site arise from downward mixing of more photochemically processed air aloft, as the nocturnal inversion breaks up in the morning. We estimate that, in the daytime, typically 1--2% of the NOy at this NOx-rich site is composed of isoprene nitrates. ¿ 1998 American Geophysical Union