The steam extraction was used to investigate the process parameters important in the extraction of essential oils from the leaves of the Eucalyptus tree. The diffusion process and the application of Fick's law, as well as the temperature dependence of the diffusion coefficient were exhibited. This has enabled the setting up of a model that takes account of the initial amount of oil in the leaves and the effect of temperature. From the literature, it can be seen that the oil collected after an extended time represents all the oil that is present in the material initially. The research conducted indicates that the extraction temperature has a significant effect on the percentage of the initial oil that is eventually extracted, with the amount of oil extracted apparently decreasing for lower temperature. This effect could, however, be modelled accurately by simply considering the temperature dependence of the diffusion coefficient. This model can lead to the design of an optimum steam extraction plant.