Process for dyeing or printing glass fibers, in particular textile glass fibers in various forms, by dyeing or printing the fiber materials with anionic dyes in the presence of cationic compounds following an acid pretreatment. The cationic compounds may be applied to the fiber material before the actual dyeing or printing process or else be applied together with the dye from the same dyeing liquor or printing paste. Suitable cationic compounds are in particular polyquaternized ammonium compounds. Level dyeings are obtained on the glass fiber materials with good light fastness properties.