Volume concentrations of steady-state secondary organic aerosol (SOA) were measured in several hydrocarbon/NOx irradiation experiments after passing through a constant temperature heated tube. Higher temperatures resulted in greater loss of particle volume. Negatives of the measured effective enthalpies of vaporization (ΔHvap,eff25--300) for SOA range from 11 to 44 kJ mol-1, and depend upon the reactant hydrocarbon (α-pinene, toluene, or 1,3,5-trimethylbenzene). Significant negative correlations between ΔHvap,eff25--300 and x> are observed for SOA formed from α-pinene or toluene. The ΔHvap,eff25--300 of SOA formed in the photooxidation of toluene is similar to that generated by the nebulization of glyoxal, indicating that the oligomers formed by nebulizing glyoxal may represent a surrogate for the chemical mixture formed in toluene photooxidation. These laboratory measurements suggest that the thermodynamic behavior of SOA depends upon the reactant hydrocarbon mixture and NOx concentration.