Self-supporting molybdenum sulfide samples with high surface area were prepared by in situ thermal decomposition in vacuum of ammonium thiomolybdate. BET specific surface area of the resulting material has its highest values of about 120m²/g for samples treated in situ at 473K and decreases on further heating, while the composition of samples treated at 423-473K is close to MoS₃, and after heating up to 573K corresponds to MoS₂. FTIR spectra show that precursor decomposition is almost completed already after treatment at 423K, although the sample still contains admixed ammonium ions. After 473K most of contaminations are gone. Spectra of adsorbed NH ₃, ND₃, CO, SO₂, CO₂, COS and CH₃SH adsorbed on samples pretreated at 423-473K are very close to those obtained for air-contacted MoS₂ powder. However higher band intensities and the use of labeled compound enabled us to identify the bands of coordinately bonded NH₃ molecules and to point to the existence of acidic sites capable to protonate ammonia. Molecularly adsorbed CH₃SH could be distinguished from its dissociative form; this reveals the existence of acid-base pair sites on bulk molybdenum sulfide.