Abstract A box model is developed to describe why the gas-phase concentrations of certain semi-volatile organic compounds (SOCs) can exhibit annual cycles that peak in the summer. Decreasing partitioning to materials at the Earth's surface with increasing temperature ( T ) is invoked to account for the summer increase. The gas-surface partition coefficients is formulated as K m ≅ K m e Qm/RT . Within a given compound class, K m is assumed to be largely independent of both compound and T ; the compound dependence in K m is found in the enthalpy of desorption from surface materials, Q m (kJ mol −1 ). For SOCs whose summer concentrations are only somewhat larger than their winter concentrations, the model predicts that the summer concentrations can give a direct estimate of q/V , the total mass of readily exchangeable SOC per unit volume of the box. The annual cycle in the gas-phase concentration for such a compound can then be analysed in search of best-fit values for Q m for the compound, and K m for the compound class. The model can then be applied to less volatile SOCs in search of their best fit values of q/V . Multiplication of the q/V values by the effective height of the troposphere gives the readily exchangeable mass per unit area of the box, q/a . Multiplication of the q/a values by an estimate of the area of the box in turn gives estimates of q . The model is applied for the first time using the data of Hoff et al. (Envir. Sci. Technol. 26 , 266–275, 1992) for samples collected at Egbert, Ontario, from July 1988 to September 1989. Highly preliminary estimates of q/V and q/a for a latitude range involving Egbert are obtained for α-HCH, p , p '-DDT, o , p '-DDT, p , p '-DDE, PCB-52 and PCB-101.