A theoretical model for the retention of organic contaminants in porous media is developed to predict the magnitude of the retention and to identify the parameters governing the retention process. The theoretical approach is based on the principles of capillary retention and is applied to porous media of equal spherical particles. The retention process is found to be affected by system parameters such as wettability, particle size and porosity. It is also found that the retention is significantly affected by the pore geometry. These findings explain available experimental results which previously appeared conflicting with regard to the effect of parameters such as particle size and porosity on the retention. Depending upon the medium pore geometry, the model predicts that the retention either increases or remains unaffected with a decrease in particle size. Also, over some ranges of medium porosity, the retention is predicted to increase with medium porosity while the reverse trend is found over other ranges of porosity. The predicted trends of behavior for the retention are compared with available experimental results and a good agreement is observed. For a completely wetting fluid and over a wide range of porosity and particle size, the model predicts that the retention is generally in the range of 12–24% of pore volume.