A process for the disproportionation of toluene over a nickel-modified mordenite catalyst which has been pretreated with mild sulfiding procedure. The sulfur dose is employed in a minor amount relative to the nickel content of the catalyst. The modified mordenite catalyst is contacted with a sulfur-containing compound such as hydrogen sulfide or dimethyldisulfide (DMDS) under pretreatment conditions involving a temperature of at least 100° C. The sulfur-containing compound is employed in a relatively small amount to passivate only a minor portion of the active nickel sites. A toluene-containing feedstock is brought into contact with the pretreated catalyst under conditions effective for the disproportionation of toluene and a disproportionation product is removed from contact with the catalyst. The mordenite catalyst contains nickel in an amount within the range of 0.1-2 wt. %. The catalyst may contain another metal such as palladium or platinum, or a lanthanide series metal such as lanthanum or cerium. Pretreatment of the catalyst may be carried out by flowing a fluid pretreatment stream having a sulfur component into contact with the catalyst. The pretreatment stream comprises hydrogen sulfide or a thio compound which is converted to hydrogen sulfide under the pretreatment conditions.