Oxidation of isoprene, the dominant non-methane biogenic volatile organic compound emitted into the atmosphere, is a significant source of secondary organic aerosol (SOA). Here formation of SOA from isoprene oxidation by the hydroxyl radical (OH) under high-NO conditions is measured. This work improves our understanding of isoprene SOA formation by demonstrating that low-volatility compounds formed under high-NO conditions produce significantly more aerosol than previously thought.