Continuous syntheses of performic and peracetic acids were performed in a microstructured reactor. Reactants were corresponding carboxylic acids and hydrogen peroxide. Sulfuric acid was used as a homogeneous catalyst. In the experiments, formation rates of performic and peracetic acids were studied at different temperatures and catalyst concentrations. Both reactions seemed to be rather fast because with performic acid equilibrium was reached in 4 min at 313 K and with peracetic acid in 10 min at 343 K. The information achieved was used in the preliminary design of a production scale on-site reactor with the plate structure. The reactor concept consists of reaction and heat transfer plates which alternate in the unit. The design is rather simple, compact and flexible in terms of capacity producing max. 100 t/a performic acid and max. 170 t/a peracetic acid. (C) 2010 Elsevier B.V. All rights reserved.