Abstract During tertiary miscible gas injection direct contact between gas and oil can be prevented by water surrounding residual oil. The principal aim of our study is to assess the importance of this waterblocking phenomenon in multicomponent gas injection. We study this process using a multicomponent pore-scale model. Light components in the gas dissolve in the water and diffuse through the water to reach the oil. This causes the oil to swell. Eventually the oil swells sufficiently to contact the gas directly. However, components in the oil can diffuse into the gas, causing the oil to shrink and preventing the contact. We apply our model to a variety of first-contact and multiple-contact miscible gas/oil systems from published field studies. Due to the low solubility of hydrocarbons in water, oil swelling and shrinkage can prevent direct contact for many days to years. We show that increasing the miscibility of injected gas, by, for instance, moving from a multi-contact miscible to a first-contact miscible displacement increases the time taken to achieve direct gas/oil contact. This leads to an extended two-phase region in the reservoir, even for a thermodynamically miscible gas flood.