The photolysis of Cl-2 molecules in the presence of toluene and oxygen, at levels of similar to 10(14) radicals/cm(-3), initiates a sequence of chemical reactions that rapidly produce an aerosol. Size distributions of the aerosol particles are examined, using a scanning mobility particle sizer, as a function of time, photolysis energy, the initial concentrations of toluene and chlorine, and of added NO and HO2. The number of particles and the volume of aerosol both exhibit a steep nonlinear increase as the initial chlorine atom level is raised. Surprisingly, the number of particles displays a strong inverse dependence on the initial toluene concentration, whereas the aerosol volume remains nearly unaffected by toluene level. Kinetic measurements of particle formation made using a flow reactor reveal an incubation period after initiation of the Cl + C6H5CH3 reaction, followed by steep increases in particle number and volume. The particle number rapidly reaches a plateau, whereas the aerosol volume continues to increase with time. The earliest observed particles are unexpectedly large, with mean diameters as high as 100 nm; a continuous growth from <10 nm is generally not observed. Both NO and HO2 suppress aerosol formation. These observations prompt us to postulate a mechanism whereby a minor reaction channel between chlorine atoms and benzylperoxy radicals to produce a Criegee intermediate controls the number of critical nuclei. This rate-limiting step is followed by rapid condensation of semivolatile compounds onto the nuclei. Because the aerosol volume can represent 10%, or more, of the toluene consumed, this necessarily includes products from the major oxidation pathways. As part of this work, we report 295 K rate constants of k(4) = (8 +/- 2) x 10(-12) cm(3) s(-1) for the benzylperoxy self-reaction, and k(6) (2.7 +/- 0.5) x 10(-11) cm(3) s(-1) for its reaction with NO.