A chromatography or supercritical extraction method is disclosed, in which the eluent comprises a mixture of a fluid and a modifier and in which the fluid is recycled. One exemplary method comprises an operation consisting in determining at least one quantity linked to the level of modifier that is mixed with the recycled fluid and, if necessary, a correction operation in order to limit variations in the level of modifier in the eluent at the inlet of the column or the extractor. The disclosure also relates to a chromatography or extraction installation.