One exposure pathway which has a significant influence on the outcome of a site specific risk assessment when volatile contaminants compounds are involved, is the leakage of contaminated subsurface gases into the indoor air environment. Evaluation of this pathway largely is completed with the use of mathematical models. The current level of understanding is that both convective and diffusive contaminant transport processes in the soil and across the subsurface building envelope will contribute to degraded indoor air quality. The convective driving force for soil gas infiltration is present because of a differential pressure between the indoor air and the subsurface soil pores. Diffusive transport occurs when low pressures are present; movement of contaminants occurs primarily due to concentration differences. Convective transport of contaminants by far creates greater impacts to the indoor air quality. Because good modelling efforts require that all transport processes and uncertainties be well understood, and documented, the aim of this work was to define situations where elevated soil gas pressures are most likely to occur, how large they can become. The project was carried out in two phases. In the first phase, soil gas pressures were monitored at several sites for a minimum period of six weeks. Following this work, a new mathematical model was developed to augment the current models in the literature. The new model was aimed at defining the rate of soil gas influx in situations where variations in the stratigraphy could cause increased convective transport.