Surface crystallization in alcohol melts results in the formation of a crystalline bilayer at the melt's surface above the freezing temperature. We demonstrate here that the structure of this surface-frozen bilayer can be varied using specific bulk additives, water and diols. Surface-specific x-ray methods show that water swells the bilayer by intercalating into its centre at a molecular water:alcohol ratio of ∼1:2. It also induces surface freezing at chain lengths not showing the effect when dry. Diols are found to induce a reversible monolayer-bilayer surface phase transition. The surface phase diagram is determined and found to depend on the diol's chain length and concentration, in addition to temperature. The physics underlying the phase behaviour is discussed.