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-H₂ 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.