Method of adjusting the viscosity of a salt having a melting point of less than 100° C. at atmospheric pressure (referred to as ionic liquid for short), wherein an oligomeric or polymeric compound which has a content of at least 0.01 mol of covalently bound acid groups/100 g of compound with at least 1% of the acid groups being present as a salt with an organic cation comprising at least one heteroatom selected from among nitrogen, oxygen, sulfur and phosphorus is added to the ionic liquid.