Process for dyeing or printing unmodified polyolefin fibers with vat dyestuffs, leuco esters of vat dyestuffs or sulfur dyestuffs, by reacting under high-temperature dyeing conditions on these textile materials aqueous liquids or printing pastes containing the said dyestuffs in unvatted or vatted-up state or as vat acid, in the presence of a dispersion system on the basis of polyglycol ethers and, if required, converting the vatted products applied by reoxidation into their pigment form.