Aromatic aldehydes may be produced by Pseudomonas putida ATCC 12633 via biotransformation of benzoylformate and substrate analogues using the mandelic acid pathway. A reduction in pH to 5.4 causes the partial inactivation of the benzaldehyde dehydrogenase isoenzymes within this pathway, thereby allowing the accumulation of the aldehyde. The reduction in dehydrogenase activity with pH is a general phenomenon affecting all dehydrogenases within the cell and is not restricted to the benzaldehyde dehydrogenases of the mandelate pathway. Both benzaldehyde and thiophene-2-carboxaldehyde have been produced with yields of over 85%.