The present invention provides a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, wherein said process comprises: (a) separating off-gas generated by said oxidation reaction into an organic solvent-rich liquid stream and a water-rich vapour stream, wherein the water-rich vapour stream comprises organic compounds and non-condensable gases, and wherein said separating is performed in a pressurised distillation stage comprising a first pressurised distillation column and a second pressurised distillation column, and (b) condensing said water-rich vapour stream exiting the pressurised distillation stage into a condensate stream and an overhead gas stream in a condensing stage, characterised in that a first water-rich vapour stream is withdrawn from said first pressurised distillation column and passed to said second pressurised distillation column and a first organic solvent-rich liquid stream is withdrawn from the first pressurised distillation column and returned to the oxidation reaction, wherein said process further comprises the step of monitoring the oxidation reaction by analysing the composition of said first water-rich vapour stream.