Abstract: A Diffusion cell for use in monitoring levels of gases or volatile organic compounds within bulk media such as ground water, effluent, or air. The diffusion cell is of sealed hollow construction and has a membrane providing an interface between a gas phase contained within the diffusion cell and the bulk media. The membrane is permeable to gas and volatile organic compounds while being relatively impermeable to water (i.e. hydrophobic). The diffusion cell includes a core of porous material to minimize the internal volume within. In use, a sensor is located in close proximity to the core of porous material to minimize the dead volume between the sensor and the diffusion cell. The minimization of the internal and dead volumes allows for decreased response times to fluctuations in levels being measured, lending the diffusion cell to real time monitoring applications.