Abstract: A process for producing aromatic aldehydes by carbonylation of an arene in the presence of triflic acid catalyst is described by reacting an arene, such as toluene, with carbon monoxide in the presence of triflic acid (as a catalyst), for a reaction time less than 90 minutes, at a carbon monoxide partial pressure of about 700-2000 psig, at a temperature of from about 0-50.degree. C. to produce an aromatic aldehyde such as tolualdehyde and by-products, the by-products being less than 5% by weight based on the total reaction products.The mole ratio of triflic acid to arene is between about 0.8 to 20, with the reaction having an isomer selectivity greater than 95% by weight for the para isomer aldehyde, and showing substantially no meta isomer.