We examined a new method to determine the aerosol yield of precursors of secondary organic aerosols in the presence of organic seed particles. To distinguish between the oxidation products of the compound in question and the organic seed, the compound was labeled with stable isotopes and aerosol samples were analyzed by isotope ratio mass spectrometry (IRMS). ¹³C labeled isoprene was obtained from isoprene emitting plants that were exposed to ¹³CO₂. The aerosol yield of isoprene was determined from the ¹³C/¹²C ratio measured in the aerosol. Measurements at organic aerosol mass concentrations as low as 10 μg m^-3 were performed. Three different methods of aerosol sampling procedures were evaluated: impactor, filter, and electrostatic deposition. The excess-% ¹³C measured by the three sampling methods agreed well. The aerosol yield of isoprene derived from these measurements showed a strong dependence on further oxidation of first-generation products and is within the range of reported yield values (1-5%) obtained so far from pure isoprene experiments.