In order to ensure the practical application of a titanium dioxide-based photocatalytic system, it is essential that reactors capable of being scaled up be designed. This has been difficult due to the issue of light distribution and irradiation of the photocatalytic surface. In this paper, a novel design utilizing a mixture of glass granules and titanium-dioxide coated particles are used in a packed bed configuration. A detailed phenomenological model to predict degradation rates of gas phase organic species (toluene) is developed. The parameters of the model are evaluated by experiments performed in differential bed reactors. These are then used in the model and the predictions are compared to the measured values, and reasonable agreement is obtained. The surface area utilization factor is approximately 0.66, indicating that a large fraction of the surface is being irradiated. These configurations can therefore be scaled up for use in larger scale applications.