Olefin polysulfide compositions of high sulfur content and of very low chlorine content are obtained by a process comprising the steps of: (1) reacting sulfur mono- and/or dichloride with at least one aliphatic monoolefin of 2-12 carbon atoms, to form an addition product or "adduct", (2) reacting hydrogen sulfide with an alkali hydroxide or ammonium hydroxide dissolved in at least one substantially anhydrous C.sub.1 -C.sub.4 aliphatic monoalcohol, with optional addition of elemental sulfur, (3) contacting said "adduct" together with at least one monohalogenated hydrocarbon compound, with the alcoholic solution obtained at the end of step (2), (4) heating the resultant mixture from step (3), removing the monoalcohol, while adding water, and (5) after settling and removal of the aqueous phase, recovering the organic phase formed at least in major part of the desired olefin polysulfide composition. These compositions, of sulfur content up to 65% by weight and of chlorine content generally lower than about 0.1% by weight, are used in particular as additives for improving the extreme-pressure properties of lubricants.