The present invention relates to a method of dyeing a substrate with a reactive dyestuff in supercritical or near supercritical carbon dioxide, said substrate being selected from the group consisting of cellulose fibers, modified cellulose fibers, protein fibers and of synthetic fibers, or any combination thereof, wherein the method comprises the subsequent steps of: pre-treating the substrate by wetting the substrate with a fluid medium containing at least 10 wt. %, preferably at least 40 wt. % of one or more organic hydrogen bond acceptor compounds selected from the group consisting of C1-C6 alkanols, dimethyl sulfoxide, dimethylformamide, acetone, butan-2-one, dimethyl ether, methyl acetate and ethyl acetate; dyeing the substrate by contacting the pre-treated substrate with supercritical or near supercritical carbon dioxide containing a reactive dyestuff.