Timescales for secondary organic aerosols (SOA) to reach equilibrium were estimated under various temperatures and relative humidities. Equilibration timescales in free troposphere can be longer than hours or days, even at moderate or relatively high relative humidities. These results provide critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas- and particle-phase concentrations of semi-volatile compounds in chemical transport models.