A theoretical model for dry sorption injection is developed. The model uses a transient diffusion equation for particles with adsorption represented using a Freundlich isotherm, and a balance equation for the bulk gas phase. The case of injection of adsorbing particles having a log-normal distribution of sizes is considered. It is found that for the conditions of practical interest just three parameters determine the adsorption behaviour. These are the exponent, q, determining the non-linearity of the Freundlich equilibrium relation; the non-dimensional adsorbate capacity of the particles, K*phi (T), where K* represents capacity per unit mass of particle and phi (T) the mass of particles injected; and the geometric standard deviation, ap, Of particle size in the particle population. Comparison is made between the model and existing experimental data for single particles under conditions of uniform external adsorbate concentration. The data span a range of particle geometries, sizes and adsorbate concentrations and the model is found to reproduce the entire range of data to within about +/-20%. The parameters of the model are systematically varied in a series of computations that illustrate and confirm the theoretical analyses.