A mixed-phase relative rates approach for measuring rates of reaction in aerosols is presented. Using this method the rate of reaction of methyl oleate (MO) particles, normalized to the gas-particle collision rate, was measured to be γMO = 1.12 (¿0.36) ¿ 10-3 with 2-methyl-2-butene as the gas-phase reference. This value compares favorably with our previously published value of 1.23 ¿ 10-3 measured using an absolute technique. Reaction of bis(2-ethylhexyl) sebacate (BES) particles with Cl and OH radicals was also studied using acetone and hexanal, respectively, as the gas-phase references. The rates of reaction of BES, normalized to the gas-particle collision rate, were measured to be γBES = 1.8 (-0.3+0.8) and γBES = 2.0 (-0.1+0.6) with Cl and OH, respectively. These fast rates of reaction (γBES > 1) imply that secondary reactions, perhaps involving radical chain mechanisms, could impact the rate at which organic particles are oxidized in the atmosphere.