A process for improving the sun protection factor (SPF) of cellulosic fibre materials and blends thereof, which comprises contacting said materials with at least one compound of the formula (1) in which M is hydrogen, an alkali metal atom, ammonium or a cation formed from an amine; R1 and R1′ independently of each other are hydrogen, hydroxy, optionally substituted alkyl, 4-morpholinyl, —NH—R3, —N(R3)2 or —O—R3, wherein R3is optionally substituted alkyl or optionally substituted aryl, R2 is a group of formula (2) wherein X is —O— or —NH— and R4 is C1-C4-alkyl which carries at least one hydrophilic substituent and R2′ has the meaning of R1 or R2.