Molecular dynamics simulations are performed to study fluid structure and aggregation phenomena in n-decane/water systems confined in hydrophobic carbonaceous pores in the presence of octane-1,2-diol, octane-4,5-diol or octane-1,8-diol. The three additives demonstrate a common trend towards self-aggregation, but differ significantly in the structural features of the resulting aggregates and in the distribution of diol molecules relative to the liquid–liquid and liquid–solid interfaces. According to simulation data, octane-1,8-diol forms the best-ordered structures upon self-aggregation and shows a noticeable effect on the morphology of the liquid–liquid interface between confined water and hydrocarbon.