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 120 m 2 /g for samples treated in situ at 473 K and decreases on further heating, while the composition of samples treated at 423-473 K is close to MoS 3 , and after heating up to 573 K corresponds to MoS 2. FTIR spectra show that precursor decomposition is almost completed already after treatment at 423 K, although the sample still contains admixed ammonium ions. After 473 K most of contaminations are gone. Spectra of adsorbed NH 3 , ND 3 , CO, SO 2 , CO 2 , COS and CH 3 SH adsorbed on samples pretreated at 423-473 K are very close to those obtained for air-contacted MoS 2 powder. However higher band intensities and the use of labeled compound enabled us to identify the bands of coordinately bonded NH 3 molecules and to point to the existence of acidic sites capable to protonate ammonia. Molecularly adsorbed CH 3 SH could be distinguished from its dissociative form; this reveals the existence of acid-base pair sites on bulk molybdenum sulfide.