Matrix diffusion is an important transport process in geologic materials of low permeability - such as most unweathered and compact igneous rocks. For predicting the fate and transport of contaminants or ground water tracers, a detailed understanding of the complexity of diffusion processes in these rocks is essential. In this study, diffusive tracer transport (iodide) was investigated in three different types of igneous rocks. The selected igneous rocks represent (1) a heavily contaminated Superfund site in New England, (2) a potential nuclear waste storage site in Nevada, and (3) a site in southern Arizona at which a ground water tracer test has been conducted recently. Porosity, mineralogical composition, and other rock properties were determined. The effective diffusion coefficients and tortuosity factors were measured using the time-lag method. Further, the experiments were evaluated mathematically using an analytical solution to Fick's second law of diffusion. The results of the diffusion experiments indicate that there is a relationship between porosity and the effective diffusion coefficient of a rock. Consequently, the tortousity factor can be expressed as a function of total porosity. These observations are in good agreement with findings reported in the literature. However, sample heterogeneities, e.g. the presence of micro-fissures in selected rock samples, were suspected to have caused some deviation from the observed relation between porosity and diffusivity. Increasing the sample size and/or the number of diffusion experiments may be the best way to minimize the effects of rock heterogeneity. In general, these experiments appear to be a simple way to determine tortousity and the effective diffusion coefficient from easy to determine rock porosity values.