The displacement of oil by water from collectors containing clay which swells has been investigated theoretically and experimentally. Swelling of clay due to a change in the ion composition of water filling a stratum can influence the displacement process not only by changing the permeability, as assumed in an earlier paper [1], but also directly by changing the pore space. A modification of the theoretical scheme of the displacement of oil by a solution of an active additive is constructed to take into account these effects; the structure of the displacement front is investigated and the experimental results are analyzed.