The so-called plug & play reactor is a novel reaction device with exchangeable reaction segments as well as modules for heating/cooling and mixing. Its performance is demonstrated for two model processes: the production of acetylsalicylic acid with a fixed bed of commercial ion-exchange particles and a Suzuki-Miyaura cross-coupling with a solid Pd-catalyst developed by us. The reaction progress was monitored by inline UV-vis spectroscopy and compared with offline HPLC. The novel set-up leads to quantitative yields and selectivity as well as to an improved practicability of the processes.