The racemization of N-acetylindoline-2-carboxylic acid in p-xylene revealed beneficial rate enhancements due to microwave effects, by comparing conventional and microwave heating. The magnitude of this effect was governed by the degree of heterogeneity of the reaction system. The amount of catalyst, the temperature and the amount of cosolvent played a decisive role. The microwave effect completely vanished when a homogeneous solution was heated. During the microwave-assisted heterogeneous racemization of N-acetylphenylalanine in p-xylene a comparable microwave effect was observed. The microwave effects could be rationalized by adapting selective heating in the phase boundary region of solid and liquid. Additionally, a straightforward translation was achieved from batchwise operation in a stirred reactor to a batch-loop reactor. The (heterogeneous) racemization of N-acetylindoline-2-carboxylic acid retained its microwave effect in the loop reactor. Our results demonstrated that avoiding plugging of the tubular reactor is a severe challenge in scaling up.