Co-ions are believed to have a negligible effect on surfactant adsorption, but we show here that they can significantly affect the surfactant adsorption at the air-water interface. Sum frequency generation vibrational spectroscopy (SFG) was employed to examine the effects of three halides (Cl(-), Br(-) and I(-)) on the adsorption of an anionic surfactant, sodium dodecyl sulphate (SDS), at the air-water interface. The SFG spectral features of both the interfacial water molecules and the C-H vibrations of the adsorbed surfactant alkyl chains were analysed to characterize the surfactant adsorption. We demonstrate and compare the effects of the three halides, as well as explain the unusual effect of Br(-), on the interfacial SDS and water molecules at the air/aqueous solution interface. It was observed that each of the three co-ions has a unique effect on the adsorption and conformation of the interfacial surfactant molecules at low halide concentrations of 10-50 mM, when the effect of halides on the interfacial water structure is believed to be negligible. This discovery implies that not only do they influence surfactant adsorption indirectly via the interfacial water network but also that there is an interaction occurring between these co-ions and the negatively charged head-groups at the interface via hydration by the interfacial water molecules. Even though this interaction/competition is likely to occur only between the surfactant head-groups and the halides, the surfactant hydrophobic tail was also observed to be influenced by the co-ions. These observed behavioural differences between the co-ions cannot be explained by the variation of charge densities. Therefore, further studies are required to determine the mode of action of halides influencing the adsorption of surfactant which gives Br(-) such a unique effect.