Secondary organic aerosol (SOA) formed from partitioning of oxidation products of anthropogenic and biogenic volatile organic compounds (VOCs) accounts for a substantial portion of atmospheric particulate matter. In describing SOA formation, it is generally assumed that VOC oxidation products rapidly adopt gas‐aerosol equilibrium. Here we estimate the equilibration timescale,τeq, of SOA gas‐particle partitioning using a state‐of‐the‐art kinetic flux model.τeqis found to be of order seconds to minutes for partitioning of relatively high volatility organic compounds into liquid particles, thereby adhering to equilibrium gas‐particle partitioning. However,τeqincreases to hours or days for organic aerosol associated with semi‐solid particles, low volatility, large particle size, and low mass loadings. Instantaneous equilibrium partitioning may lead to substantial overestimation of particle mass concentration and underestimation of gas‐phase concentration. Key Points Equilibration timescale of SOA partitioning is estimated Kinetically limited vs. quasi‐equilibrium particle growth is discussed Phase state has large impact on SOA partitioning kinetics