The present invention concerns a method for preparing an alkali metal sulfide, from at least one oxygenated alkali metal compound comprising at least one step a) involving reacting said oxygenated alkali compound(s) with at least one sulfur compound of formula (I): in which: R represents a linear or branched alkyl or alkenyl radical, containing 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms; n is equal to 0, 1 or 2; x is equal to 0 or to a whole number having a value of between 1 and 10, preferably x is a whole number equal to 1, 2, 3 or 4; R′ represents a linear or branched alkyl or alkenyl radical, containing 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms or, only if n=x=0, a hydrogen atom; or indeed R and R′ can form, together and with the sulfur atom(s) bearing them, a sulfur heterocycle containing 2 to 12 carbon atoms, preferably 2 to 8 carbon atoms, and optionally one or a plurality of heteroatoms chosen from oxygen, nitrogen and sulfur.