It can be produced by heating KSO with carbon (coke):
:KSO  +  4 C  →  KS  +  4 CO
In the laboratory, pure KS may be prepared by the reaction of potassium and sulfur in anhydrous ammonia. 
Sulfide is highly basic, consequently KS completely and irreversibly hydrolyzes in water according to the following equation:
:KS  +  HO  →  KOH  +  KSH
For many purposes, this reaction is inconsequential since the mixture of SH and OH behaves as a source of S.  Other alkali metal sulfides behave similarly.