Surface properties of unsupported MoS2 have been studied by means of IR spectroscopy of a wide range of adsorbed molecules. After contact with air, surface of MoS2 is partially oxidised and contains adsorbed water, Mo=O groups, sulphate and thiosulphate-like anions. These species are removable by treatment in vacuum or in H2S-H-2 mixture. On MoS2, only molecular adsorption was established for SO2, CO2, COS and CH3SH showing weak basicity of surface sulphur ions. Unsupported MoS2 presents also very weak acidic properties, as revealed by pyridine, 2,6-dimethylpyridine and ethylene adsorption. However, ammonia could be protonated or coordinately bound to MoS2 surface. CO and NO interact with the coordinately unsaturated Mo sites of the sulphide, providing evidence for an important back donation effect. Comparison with surface properties of supported MoS2 points to the lower acidic and higher metallic character of Mo sites of the unsupported sulphide.