Soil water dynamics at an experimental hillslope site is studied by means of a one-dimensional dual-continuum model. The model is based on Richards' equation for vertical soil water flow and the advection–dispersion equation for transport of the stable isotope 18O. The water body contained in the soil-matrix pore space and the one transmitted through the system of preferential pathways are treated as two separate, mutually communicating soil water continua. The 18O isotope, monitored in precipitation, subsurface hillslope discharge, and soil water, was used as a natural tracer to study the role of preferential flow in the formation of shallow subsurface runoff. It is shown that the dual-continuum approach can, in principle, explain the observed variations of 18O content in the subsurface hillslope discharge. The model successfully describes mixing of new water, which reflects the isotope signatures of the individual precipitation events, with old water, reflecting the seasonal variability of the isotope signal.