The reaction of ozone and isoprene has been studied to examine physical and chemical characteristics of the secondary organic aerosol formed. Using a scanning mobility particle sizer, the volume distribution of the aerosol was found in the range 0.05--0.2 ¿m. The aerosol yield was estimated to be 0.01, a value which is a factor of 5--10 higher than previous reports. The aerosol formation is complicated by the presence of minor impurities in the isoprene and the fact that OH-radicals produced in the ozonolysis can react with isoprene to produce organic aerosol. Without an OH-radical scavenger present, up to 50% of the observed aerosol comes from the OH channel. A GC-MS analysis of the products of the composite aerosol showed that two methyl tetrols and 2-methylglyceric acid are formed which can be attributed to the OH reaction channel. A measurement of the effective enthalpy of vaporization using a volatility differential mobility analyzer found the aerosol to have ΔH eff of -42 kJ mol-1, a value at the upper end of the range of organic aerosols previously studied. Even with the increased yield found in this study, the ozonolysis reaction probably remains a minor contributor to secondary organic aerosol in PM2.5 from the atmospheric oxidation of isoprene.