The main colouring compounds of the dye plant weld (Reseda luteola L.) are the flavones luteolin (lut), lut-7-O-glucoside and lut-7,3ʹ-O-diglucoside. Alum (an aluminium salt)-premordanted wool dyed with weld leads to yellow colours that are of low resistance to light. The photo-stability of lut in aerated methanol–water 8:2 (v/v) solution upon irradiation with light above 300 nm was studied at different lut–Al3+ ratios. Experiments using extracts of weld to dye wool premordanted with increasing quantities of aluminium salts were also carried out. The photo-stability of lut in the polar protic solvent and the photo-resistance (light-fastness) of the colour of weld-dyed wool decrease with increasing concentrations of aluminium ions. Thus, the lower the [Al3+] used for mordanting the wool, the more light-fast its colour. Lowering the [Al3+] appears to have no negative influence on the wash-fastness of the colour. As the gain in light-fastness by the use of low [Al3+] to premordant the wool is not extensive, however, this does not seem to be a way to meet today's requirement of light-fastness of the colours of dyed textiles by itself. Nevertheless, it may be part of a bigger strategy to address the need for increased light-fastness of the colour of wool dyed with weld. Implementation of this approach by dyers is expected to clarify whether it results in benefits for textile dyeing practice.