Process for formylating aryl halides

In a process for formylating optionally substituted iodo- bromo- and/or chloro-aryl compound with carbon monoxide and hydrogen at a pressure from 20 to 400 bars and at a temperature in the range of 80.degree. to 250.degree. C. in the presence of a noble metal catalyst and a tertiary organic nitrogen compound, the improvement which comprises carrying out the formylation in the presence of a phosphane and/or phosphite, said phosphane and/or phosphite present in an amount of 10 to 1,000 times the molar amount of catalyst.