A reactor (1) has lower reactor region (2) for accommodating sulfur melt (3), non-pressure-bearing caverns (4) for accommodation of product gas mixture which forms in exothermic reaction and contains sulfur, supply device (5,5a) for controlled supply of pressurized gaseous hydrogen per cavern, gas-collecting region (6) for accommodating the product gas mixture, non-pressure-bearing installed device(s) (7) for transfer of the total amount of product gas mixture formed in the lower reactor region to the gas-collecting region and catalyst in the non-pressure-bearing installed device(s). A reactor has lower reactor region for accommodating sulfur melt, non-pressure-bearing caverns for temporary accommodation of product gas mixture which forms in exothermic reaction and comprises hydrogen sulfide, sulfur and hydrogen, at least one supply device for controlled supply of pressurized gaseous hydrogen per cavern, gas-collecting region for accommodating the product gas mixture at elevated temperature and elevated pressure with respect to standard conditions, non-pressure-bearing installed device(s) for continuous transfer of the total amount of product gas mixture formed in the lower reactor region to the gas-collecting region and catalyst in the non-pressure-bearing installed device(s), for reaction of sulfur and hydrogen maintained in the product gas mixture to hydrogen sulfide. An independent claim is included for preparation of hydrogen sulfide.