Measurements of atmospheric organic nitrates derived from isoprene, i.e., isoprene nitrates, were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on-line chromatography in conjunction with a nitrate-selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy, which was typically 0.5--1.5% of total odd nitrogen, but up to ~4% for well-aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation. ¿ 2001 American Geophysical Union