The kinetics and products of the heterogeneous OH-initiated oxidation of squalene (C30H50, a branched alkene with 6 C=C double bonds) particles are measured. These results are compared to previous measurements of the OH-initiated oxidation of linoleic acid (C18H32O2, a linear carboxylic acid with 2 C=C double bonds) particles to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. In a 10% mixture of O-2 in N-2 in the flow reactor, the effective uptake coefficients (gamma(eff)) for squalene and linoleic acid are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. gamma(eff) for squalene is 2.34 +/- 0.07, which is smaller than gamma(eff) for linoleic acid (3.75 +/- 0.18) despite the larger number of C=C double bonds in squalene. gamma(eff) for squalene increases with [O-2] in the reactor, whereas gamma(eff) for linoleic acid decreases with increasing [O-2]. This suggests that the chain cycling mechanism in these two systems is different since O-2 promotes chain propagation in the OH + squalene reaction but promotes chain termination in the OH + linoleic acid reaction. Elemental analysis of squalene aerosol shows that an average of 1.06 +/- 0.12 O atoms are added per reactive loss of squalene prior to the onset of particle volatilization. O-2 promotes particle volatilization in the OH + squalene reaction, suggesting that fragmentation reactions are important when O-2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, O-2 does not influence the rate of particle volatilization in the OH + linoleic acid reaction. This indicates that O-2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.