The present invention relates to an improved process for the preparation of substituted 2-(4-carbonylmethoxy-optionally 2,5-disubstituted-phenyl)-acetaldehydes, in particular 2-(4-alkoxycarbonylmethoxy-optionally 2,5-disubstituted-phenyl)-acetaldehydes and their use in the synthesis of optionally substituted 2-[4-[2-[[-2-hydroxy-2-(4-hydroxyphenyl)-1-methylethyl]-amino]ethyl]-opti- onally 2,5-disubstituted-phenoxy]acetic acid derivatives or the salts thereof, which may be used as pharmaceutically active substances.